[java-spanner-jdbc] feat: support option for setting `client-id`

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.