Add authentication guide for Google Cloud APIs

Adds a guide for configuring authentication to Google Cloud APIs.

Though this provides enough information to cover the feature added in
this PR, we should improve the way to configure a
`ServiceAccount` for practical use cases since in any methods users need
to reconfigure IAM stuff every time a new `ProviderRevision` is created.

Signed-off-by: micnncim <micnncim@gmail.com>

docs/AUTHENTICATION.md

@@ -0,0 +1,162 @@
+# Authenticating to Google Cloud APIs
+
+`provider-gcp` requires credentials to be provided in order to authenticate to
+the Google Cloud APIs. This can be done in one of the following ways:
+
+- Authenticating using a base-64 encoded service account key in a Kubernetes
+  `Secret`. This is described in detail [here](https://crossplane.io/docs/v1.6/getting-started/install-configure.html#get-gcp-account-keyfile).
+- Authenticating using [Workload Identity](https://cloud.google.com/kubernetes-engine/docs/concepts/workload-identity).
+  This is described in the [section below](#authenticating-with-workload-identity).
+
+## Authenticating with Workload Identity
+
+*Note: This method is supported in `provider-gcp` v0.20.0 and later.*
+
+Using Workload Identity requires some additional setup.
+Many of the steps can also be found in the [documentation](https://cloud.google.com/kubernetes-engine/docs/how-to/workload-identity).
+
+### Steps
+
+These steps assume you already have a running GKE cluster which has already
+enabled Workload Identity and has a sufficiently large node pool.
+
+Note that you can specify any valid strings to the variables below unless the
+variable is explicitly assigned.
+
+#### 1. Install Crossplane
+
+Install Crossplane from `stable` channel:
+
+```bash
+$ helm repo add crossplane-stable https://charts.crossplane.io/stable
+$ helm install crossplane --create-namespace --namespace crossplane-system crossplane-stable/crossplane
+```
+
+`provider-gcp` can be installed with either the [Crossplane CLI](https://crossplane.io/docs/v1.6/getting-started/install-configure.html#install-crossplane-cli)
+or a `Provider` resource as below:
+
+```console
+$ cat <<EOF | kubectl apply -f -
+apiVersion: pkg.crossplane.io/v1
+kind: Provider
+metadata:
+  name: ${PROVIDER_GCP}
+spec:
+  package: crossplane/provider-gcp:v${VERSION} # v0.20.0 or later
+  # Set a controllerConfigRef if you have a ControllerConfig:
+  # controllerConfigRef:
+  #   name: ${CONTROLLER_CONFIG}
+EOF
+```
+
+#### 2. Configure service accounts to use Workload Identity
+
+Create a GCP service account, which will be used for provisioning actual
+infrastructure in GCP, and grant IAM roles you need for accessing the Google
+Cloud APIs:
+
+```console
+$ gcloud iam service-accounts create ${GCP_SERVICE_ACCOUNT}
+$ gcloud projects add-iam-policy-binding ${PROJECT_ID} \
+    --member "serviceAccount:${GCP_SERVICE_ACCOUNT}@${PROJECT_ID}.iam.gserviceaccount.com" \
+    --role ${ROLE} \
+    --project ${PROJECT_ID}
+```
+
+Get the name of your current `ProviderRevision` of this provider:
+
+```console
+$ REVISION=$(kubectl get providers.pkg.crossplane.io ${PROVIDER_GCP} -o jsonpath="{.status.currentRevision}")
+```
+
+Next, you'll configure IAM to use Workload Identity.
+In this step, you can choose one of the following options to configure service accounts:
+
+- [Option 1] Use a Kubernetes `ServiceAccount` managed by a provider's controller.
+- [Option 2] Use a Kubernetes `ServiceAccount` which you created and is specified to `.spec.serviceAccountName`
+  in a [`ControllerConfig`](https://doc.crds.dev/github.com/crossplane/crossplane/pkg.crossplane.io/ControllerConfig/v1alpha1@v1.6.2).
+
+##### 2.1. [Option 1] Use a controller-managed `ServiceAccount`
+
+Specify a Kubernetes `ServiceAccount` with the revision you got in the last
+step:
+
+```console
+$ KUBERNETES_SERVICE_ACCOUNT=${REVISION}
+```
+
+##### 2.1. [Option 2] Use a user-managed `ServiceAccount`
+
+Create a `ServiceAccount`, `ControllerConfig`, and `ClusterRoleBinding`:
+
+```console
+$ cat <<EOF | kubectl apply -f -
+apiVersion: v1
+kind: ServiceAccount
+metadata:
+  name: ${KUBERNETES_SERVICE_ACCOUNT}
+  namespace: crossplane-system
+---
+apiVersion: pkg.crossplane.io/v1alpha1
+kind: ControllerConfig
+metadata:
+  name: ${CONTROLLER_CONFIG}
+spec:
+  serviceAccountName: ${KUBERNETES_SERVICE_ACCOUNT}
+---
+apiVersion: rbac.authorization.k8s.io/v1
+kind: ClusterRoleBinding
+metadata:
+  name: crossplane:provider:${PROVIDER_GCP}:system
+roleRef:
+  apiGroup: rbac.authorization.k8s.io
+  kind: ClusterRole
+  name: crossplane:provider:${REVISION}:system
+subjects:
+- kind: ServiceAccount
+  name: ${KUBERNETES_SERVICE_ACCOUNT}
+  namespace: crossplane-system
+EOF
+```
+
+#### 2.2. Allow the Kubernetes `ServiceAccount` to impersonate the GCP service account
+
+Grant `roles/iam.workloadIdentityUser` to the GCP service account:
+
+```console
+$ gcloud iam service-accounts add-iam-policy-binding \
+    ${GCP_SERVICE_ACCOUNT}@${PROJECT_ID}.iam.gserviceaccount.com \
+    --role roles/iam.workloadIdentityUser \
+    --member "serviceAccount:${PROJECT_ID}.svc.id.goog[crossplane-system/${KUBERNETES_SERVICE_ACCOUNT}]" \
+    --project ${PROJECT_ID}
+```
+
+Annotate the `ServiceAccount` with the email address of the GCP service account:
+
+```console
+$ kubectl annotate serviceaccount ${KUBERNETES_SERVICE_ACCOUNT} \
+    iam.gke.io/gcp-service-account=${GCP_SERVICE_ACCOUNT}@${PROJECT_ID}.iam.gserviceaccount.com \
+    -n crossplane-system
+```
+
+### 3. Configure a `ProviderConfig`
+
+Create a `ProviderConfig` with `InjectedIdentity` in `.spec.credentials.source`:
+
+```console
+$ cat <<EOF | kubectl apply -f -
+apiVersion: gcp.crossplane.io/v1beta1
+kind: ProviderConfig
+metadata:
+  name: default
+spec:
+  projectID: $PROJECT_ID
+  credentials:
+    source: InjectedIdentity
+EOF
+```
+
+### 4. Next steps
+
+Now that you have configured `provider-gcp` with Workload Identity supported,
+you can [provision infrastructure](https://crossplane.io/docs/v1.6/getting-started/provision-infrastructure).