Add indefinite wait with readiness check to ensure postgres running

[m-dwyer]

Context

Sometimes, a project consuming the project may have a postgresql service with a dependency on another service, such as a setup. This causes the init_outbox to immediately fail if postgres isn't ready.

Similar to the kafka_local, we create a postgres_wait service that will tail indefinitely, and has a 3 second readiness probe to confirm postgres is ready. Once ready, init_outbox can run successfully.

Changes

Add postgres_wait service to check postgres is ready, and have init_outbox depend on this, so we don't attempt to setup postgres before ready.

[jasononeil]

Thanks!