Added abstracted infrastracture for handle custom auth provider [GOOGLE ONLY]
- The auto provider identity, if set, can be used to sign an impersonation token credential.
- The auto provider identity, if set, can be used to perform Auth Operations like credentials exchange.
- Introduced because different auth providers can be used on a CSP deployment
- The decoding policy is defined in the src/auth/auth.ts and implemented in src/auth/providers/*
- The auth provider secrets are store in GCP secretsManager and can be activated via ENV variable (google/config.ts for ref)