Allow build time secrets injection from AWS Secrets Manager

This is a request to support fetching build time secrets (such as a token to access private libraries) from AWS Secrets Manager and injecting them into the build time environment.

The main alternative I see is to somehow inject the credentials in the build command.

Another solution would be to leverage the instance profile's permissions and then to add some automation in the the Docker entrypoint to fetch the libraries at runtime. This is less ideal because it adds to the service startup time.

It would be nice if this were supported natively in App Runner.