Allow unknown properties in connection URL / properties

[olavloite]

Some applications automatically add additional properties to the Connection URL or Connection properties. Properties that are unknown to the Spanner JDBC driver will cause the driver to throw an error. This prevents the driver to be used with some applications, such as for example the IntelliJ Database Navigator: https://stackoverflow.com/questions/64975569/connecting-to-a-gcloud-spanner-emulator-instance-from-intellijs-database-naviga

The driver should therefore also have a mode where unknown properties do not cause errors but only warnings.