feat: add ability to specify extra objects to apply along with the chart

[ThatsMrTalbot]

This is useful because istio-csr depends on an external issuer to function. The ability to configure that issuer when deploying is desirable.

[SgtCoDFish]

This looks good and testing locally it works. I used the following values.yaml:

extraObjects:
  - |
    apiVersion: v1
    kind: ConfigMap
    metadata:
      name: 'some-extra-configmap'
  - |
    apiVersion: v1
    kind: Namespace
    metadata:
      name: sandbox
    ---
    apiVersion: cert-manager.io/v1
    kind: Issuer
    metadata:
      name: selfsigned-issuer
      namespace: sandbox
    spec:
      selfSigned: {}
    ---
    apiVersion: cert-manager.io/v1
    kind: Certificate
    metadata:
      name: root-ca
      namespace: sandbox
    spec:
      isCA: true
      commonName: root-ca
      secretName: root-ca
      privateKey:
        algorithm: ECDSA
        size: 256
      issuerRef:
        name: selfsigned-issuer
        kind: Issuer
        group: cert-manager.io
    ---
    apiVersion: cert-manager.io/v1
    kind: Issuer
    metadata:
      name: root-ca-issuer
      namespace: sandbox
    spec:
      ca:
        secretName: root-ca

And that correctly created all resources. I added some suggestions on rewording the doc comment but you might want to reword - I'm easy really.

What do you think?

[SgtCoDFish]

/lgtm
/approve

Cheers 🚀

[cert-manager-prow]

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