Add further libraries to spring initializr

Is there a reason why only a handful of the spring-cloud-gcp starters are featured in the spring initializr (start.spring.io)?

The data starters (datastore, firestore, spanner) and the sql starters (jdbc and r2dbc) would be really convenient and boost the discoverability.

Also, in combination with testcontainers-gcloud, new projects could be bootstrapped more quickly.

If needed, I can also contribute the definition for the initilizr config.

Start at the start.spring.io integration and locate the configuration or definitions for the existing spring-cloud-gcp starters. Add entries for the Datastore, Firestore, Spanner, JDBC, and R2DBC starters, then verify that new projects can be generated with them.

Written by the indexing model from the issue text.