Add instance and database to Spanner emulator startup

We're using the [Spanner emulator docker image](https://cloud.google.com/spanner/docs/emulator#docker) for integration testing our Spanner database. What we're currently missing is an easy way to provide the Spanner emulator with the instance and database to create. Currently this only seems possible through added scripts and/or code initializing these things after the emulator has started. This makes it very hard to integrate with other tooling like the docker-maven-plugin and our gitlab-ci setup.

We'd very much like this to be available in the emulator on image startup, maybe through environment variables. To prevent us from needing to add extra tools on top for something that feels like basic functionality.