gplay setup (auth)

Covers the auth namespace. Shared conventions are in gplay-cli-usage.

The credential

gplay authenticates as a Google Cloud service account with the Android Publisher role, invited to your Play Console. Supply its JSON key via gplay auth login (a stored Account in the OS keystore, with a file fallback in CI containers), GPLAY_SERVICE_ACCOUNT, or --service-account; precedence in gplay-cli-usage.

First-run flow

# 1. Register the service account as the active Account. --developer-id records
#    the Play Console developer account id that `gplay team` and `customapps` address.
gplay auth login --service-account ./service_account.json --developer-id <id>