initial ephemeral poc

[BBBmau]

initial work of supporting ephemeral resource service_account_token

Has the base code for supporting ephemerals which is including in the provider config itslef. Still requires some work on supprting services such as IAMCredentialsBasePath which is crucial for supporting both service_account_token and service_account_jwt

Although this PR covers all of them we should only really focus on service_account_token.

This PR can easily be closed and opened again to make the ephemeral development process much smoother.

Release Note Template for Downstream PRs (will be copied)

[SarahFrench]

Making a note: the Go rewrite was just merged into the feature branch via the sync PR. This has caused some conflicts, and the .go.erb files are no longer valid