Update GoogleAuthFilter with recommended GCP Service-to-Service Authentication approach

[jeremyg484]

Feature description

The current recommendation from Google for handling Service-to-Service authentication is to make use of the GCP credentials classes, which can more smoothly handle building the necessary OAuth request metadata. The library is able to make use of Application Default Credentials, and is thus more flexible, allowing testing of service-to-service authentication from a developer's local machine.

Our current approach uses the internal GCP metadata server. This server is only reachable from within a deployed GCP application, thus harder to validate and test.

GoogleAuthFilter should be updated to make better use of the GoogleCredentials that are already managed by the Micronaut GCP library.