feat: Add support for authentication credentials in ProviderConfig

[arielsepton]

PR Description: Add Authentication Credentials Support in ProviderConfig

Summary

This PR enhances the ProviderConfig by adding support for authentication credentials via a Kubernetes Secret reference. This feature allows users to securely manage the Authorization header for all requests made with this configuration.

Changes

• Added support in a secretRef field to reference a Kubernetes Secret containing the authentication token, which will be automatically included in the Authorization header for all requests.

Example Usage

apiVersion: http.crossplane.io/v1alpha1
kind: ProviderConfig
metadata:
  name: http-conf
spec:
  credentials:
    source: Secret
    # The value of this secret will be used to set the "Authorization" header for all requests made with this config.
    # It will be added as: "Authorization": "<secret-value>".
    secretRef:
      namespace: crossplane-system
      name: http-provider-secret
      key: token

Notes

This change is backward-compatible; existing configurations remain unaffected.
Fixes #53

I have:

• Read and followed Crossplane's [contribution process].
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