OptionalprojectGCP project id.
OptionalkeyPath to a service-account key file.
OptionalapiOverride the API endpoint (e.g. 'localhost:8085' for the emulator).
OptionalclientA pre-built PubSub client (or a mock exposing the methods above). When
provided, connect() uses it directly and does NOT require the driver —
this is how tests inject a mock client.
Connection options for
PubSubStore.