Starts the supervision tree for GcpGcs.
Two children are supervised:
GcpGcs.Auth's cache — an internal GenServer that owns the ETS table backing the access-token cacheFinch— the HTTP connection pool used for all requests
The Finch pool is sized from config :gcp_gcs, :finch and targets either the
production endpoint (storage.googleapis.com) or a configured emulator. See
GcpGcs.Config.