gapic: support EnableJwtWithScope for gRPC GAPIC clients

[shinfan]

• Update the GAPIC clients to allow scope for self-signed JWT
• The self-signed JWT is now the default authentication method for service account instead of OAuth 2.0
• Self-signed JWT AIP: https://google.aip.dev/auth/4111
• Related PRs:
  • feat(internal): [AIP-4111] support scopes for self-signed JWT auth flow google-api-go-client#1075
  • google: support scopes for JWT access token golang/oauth2#504
  • 4110: Update ADC with support for self-signed JWT aip-dev/google.aip.dev#764

[shinfan]

cc @codyoss @silvolu @bshaffer

[noahdietz]

googleapis/google-api-go-client#1075 needs to be released and we might need to update the version of google.golang.org/api used in the showcase integration test submodule. The integration tests do not compile atm because of this.

LGTM otherwise

[shinfan]

LGTM otherwise

Thanks Noah. I will fix the dependency issue once it is released.

[codyoss]

Released: https://github.com/googleapis/google-api-go-client/releases/tag/v0.49.0

[shinfan]

@noahdietz PTAL