Add support for execute(String, ... autogeneratedKey) methods

[olavloite]

The Statement interface contains a number of execute(String, ...) methods that can be used for returning generated key values. These methods currently throw an SQLFeatureNotSupported exception, but according to the specs the driver should ignore the autogenerated key hints if these are not supported by the given sql string. Cloud Spanner does not support any autogenerated keys, meaning that these methods should always return the same as its execute(String) or executeUpdate(String) equivalent.

[olavloite]

Method implementations added.