As an installer user, I want my gcp creds used for install to be used by the CAPG controller when provisioning resources.

Acceptance Criteria: 

• Users can authenticate using the service account from ~/.gcp/osServiceAccount.json
• users can authenticate with default application credentials
• Docs team is updated to whether existing credential methods will continue to work (specifically environment variables):  see official docs

Engineering Details:

• We can pass in a secret containing the service account creds, so we can do t hat during the manifest stage, which is probably a more appropriate place
• The controller supports auth'ing with default application credentials when there is no secret supplied, so that's good. That should work for use cases where they don't want service account
• GCP controller authentication is handled here: https://github.com/kubernetes-sigs/cluster-api-provider-gcp/blob/main/cloud/scope/managedcontrolplane.go#L67
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This requires/does not require a design proposal.
This requires/does not require a feature gate.