Add H2 (in-mem) and PostgreSQL database suppliers, and make the Keycloak test server supplier depend on the database.

The database will return the configuration needed for the Keycloak server to connect to it (hostname, username, password, etc.).

The H2 database supplier, will basically do nothing other than return the config options (db=dev-mem?), while the Postgres database supplier will start a Postgres testcontainer, and return configuration to the test server supplier like hostname, username, password, etc.