feat: support option for setting client-id

[olavloite]

The JDBC driver automatically creates a Spanner instance based on the JDBC connection URL. If an application uses several different JDBC connection URLs, then the JDBC driver will create several Spanner instances. Each instance will be assigned a monotonically increasing client-id that is included in the metrics that are being collected. It is hard for users to relate a given client-id to a specific connection URL. Adding an option for setting the client-id that should be used would make this easier.