Feature Request: Add support for Google Secret Manager for git secret

Hi

Config Sync is very much aimed at GCP customers and environments (Anthos). From a security perspective, in my org, k8s secrets are discouraged and use of GCP Secret Manager is a better option.

Yes there are workarounds such as [mounting to k8s via secrets-store-csi-driver](https://github.com/kubernetes-sigs/secrets-store-csi-driver).

However, we would like to be able to natively spec a GCP Secret Manager Secret as the source for Git credentials (private key/token) in the RootSync/RepoSync.

Is this on the roadmap or can be added please?