AppDistribution - Support 'external_account' WIF service account credentials - Gradle

## What feature would you like to see?

Google strongly recommends using WIF credentials instead of legacy service-account credentials.json keys. We have followed that rule religiously across all of our CI integrations with GCP. This is the first tool that does not support WIF credentials.

```
> Task :app:appDistributionUploadProdRelease FAILED
Using APK path specified by the artifactPath parameter in your app's build.gradle: /home/circleci/project/app/build/outputs/apk/prod/release/app-prod-release.apk.
Uploading APK to Firebase App Distribution...
Using credentials file specified by environment variable GOOGLE_APPLICATION_CREDENTIALS: /home/circleci/.gcp_wif_credentials.json

FAILURE: Build completed with 2 failures.

1: Task failed with an exception.
-----------
* What went wrong:
Execution failed for task ':app:appDistributionUploadProdRelease'.
> Service credentials file does not exist. Please check the service credentials path and try again: Error reading credentials from stream, 'type' value 'external_account' not recognized. Expecting 'authorized_user' or 'service_account'.
```