feat: Custom service account annotations

[jsirianni]

Description of Changes

Added support for arbitrary service account annotations.

When running helm template with annotations defined (see test update), I get the following service account.

apiVersion: v1
kind: ServiceAccount
metadata:
  name: release-name-bindplane
  namespace: default
  labels:
    app.kubernetes.io/name: bindplane
    app.kubernetes.io/stack: bindplane
    app.kubernetes.io/instance: release-name
    app.kubernetes.io/managed-by: Helm
  annotations:
    iam.gke.io/gcp-service-account: bindplane-sa

This annotation allows users to implement workload identity in GKE. You can read more about it here. https://cloud.google.com/kubernetes-engine/docs/how-to/workload-identity

Closes #177

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• CI passes
• Changes to ports, services, or other networking have been tested with istio