[java-spanner-jdbc] feat: Support CREATE DATABASE and ALTER DATABASE in JDBC

The JDBC driver supports all DDL statements that affect an existing database, but currently does not support `CREATE DATABASE`. It also does not support the `ALTER DATABASE` statement for any other database than the one that the client is connected to at that moment. This makes it difficult to create and maintain databases using JDBC. The JDBC driver should therefore support:
- Connecting to an instance without connecting to a specific database.
- Executing `CREATE DATABASE` statements
- Executing `ALTER DATABASE` statements for other databases than the one the client is connected to at that moment.
- Executing `DROP DATABASE` statements
- Listing existing databases on an instance (`SHOW VARIABLE DATABASES`)