pekko-connectors-google-common: custom credential resolution should be replaced with google-auth-library-java

## Problem pekko-connectors-google-common reimplements Google's credential resolution instead of delegating to google-auth-library-java. This is a design issue that leads to: 1. Incomplete GCP environment support The custom implementation does not correctly handle GKE Workload Identity, while `GoogleCredentials.getApplicationDefault()` works on the same Pod. Reimplementing authentication means that every change or addition to a GCP environment requires `pekko-connectors` to catch up. 2. No non-deprecated escape hatch The only way to bypass the custom implementation (`withCallCredentials`) is **deprecated** and has no replacement. **google-auth-library-java is maintained by Google, uses standard HTTP clients (no Pekko HTTP dependency), and supports all GCP environments. There is no benefit to reimplementing this.** ## Proposal Replace the custom credential resolution with `GoogleCredentials.getApplicationDefault()` from google-auth-library-java.