feat: support DATABASE statements

[olavloite]

Adds support for the following DATABASE statements in the Connection API:

• CREATE DATABASE <database_id>: Creates a database (passes the statement through to the CreateDatabase RPC)
• ALTER DATABASE <database_id>: Alters an existing database (passes the statement through to the UpdateDatabaseDdl RPC)
• DROP DATABASE <database_id>: Drops an existing database (parsed and translated into a call to the DropDatabase RPC)
• SHOW VARIABLE DATABASES: Lists the databases on the current instance (parsed and translated into a call to the ListDatabases RPC)
• USE DATABASE <database_id>: Changes the database that is used by the connection to execute queries, updates, etc.

Also adds support for creating a Connection that is not connected to a specific database, but only to a Spanner instance. This can be used to connect to an empty instance, then create a database on the instance, and then call USE DATABASE <database_id> to use the database with the connection.

This feature allows users to execute scripts like this directly through the Connection API (and thereby also through JDBC):

CREATE DATABASE db;
ALTER DATABASE db SET OPTIONS (optimizer_version = 1);
USE DATABASE db;

CREATE TABLE Singers (...);
INSERT INTO Singers (...);
SELECT * FROM Singers;

DROP DATABASE db;

Needed for googleapis/java-spanner-jdbc#457

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@3c55eb3). Click here to learn what that means.
The diff coverage is n/a.

❗ Current head 98aabd2 differs from pull request most recent head 1df208e. Consider uploading reports for the commit 1df208e to get more accurate results

@@           Coverage Diff           @@
##             main    #1143   +/-   ##
=======================================
  Coverage        ?   84.92%           
  Complexity      ?     2797           
=======================================
  Files           ?      156           
  Lines           ?    14430           
  Branches        ?     1392           
=======================================
  Hits            ?    12255           
  Misses          ?     1598           
  Partials        ?      577           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3c55eb3...1df208e. Read the comment docs.

[thiagotnunes]

I started an internal discussion in regards to the syntax of the following statements:

• SHOW VARIABLE DATABASES
• USE DATABASE <database id>

I will let you know once we have a response here, sorry for the delay.