Feature: Enable support for Google Secrets Manager

Seems that a minor change to konlet-startup would enable the use of Secrets Manager to inject environment variables directly into container from Secrets Manager. Using existing Auth credentials, one could make a call to Secrets Manager after it calls the Metadata store, get the secrets and map to environment variables. Something along lines of:

- Set an Env variable as a flag to enable Secrets Manager (or use presences of the following....)
- Set an Env variable with a list of Environment Variables to secrets name mappings
- In konlet-startup, if the above is set then make authenticated call to Secrets Manager to pull secrets and add to spec.Container.Env
- Startup container using existing process

We are going to see about creating a Pull Request with this change

Start by locating konlet-startup and tracing its authenticated Metadata store call and existing container startup process. Determine how Google Secrets Manager should be called and how the proposed enablement flag and environment-variable-to-secret mappings fit the current configuration. Done means configured secrets are fetched, added to spec.Container.Env, and the container still starts through the existing process.

Written by the indexing model from the issue text.