feat(new sink): Initial `gcp_pubsub` sink implementation (#1308)

* Initial GCP pubsub implementation

This will currently only work for one hour, as the OAuth token will
expire after 3600 seconds.

Signed-off-by: Bruce Guenter <bruce@timber.io>

* Schedule periodic token renewal

The GCP authentication token expires after 3600 seconds (1 hour), so it
needs to be periodically renewed to continue access to GCP services.

Signed-off-by: Bruce Guenter <bruce@timber.io>

* Convert pubsub sink to new request config struct

Signed-off-by: Bruce Guenter <bruce@timber.io>

* Ensure that at least one of api_key and credentials_path are set

Signed-off-by: Bruce Guenter <bruce@timber.io>

* Allow for accessing an emulator for testing

Signed-off-by: Bruce Guenter <bruce@timber.io>

* Set up the emulator in docker compose

Signed-off-by: Bruce Guenter <bruce@timber.io>

* First integration test

Signed-off-by: Bruce Guenter <bruce@timber.io>

* Health check needs only one thread, not four

Signed-off-by: Bruce Guenter <bruce@timber.io>

* Create a new topic and subscription on each test

Signed-off-by: Bruce Guenter <bruce@timber.io>

* Add test to ensure healthcheck validates the topic

Signed-off-by: Bruce Guenter <bruce@timber.io>

* Drop pure default REQUEST_DEFAULTS

Signed-off-by: Bruce Guenter <bruce@timber.io>