ResultSet.getObject throws "not implemented" exception for TIMESTAMP_WITH_TIMEZONE

[aiguofer]

Describe the bug, including details regarding any error messages, version, and platform.

TIMESTAMP_WITH_TIMEZONE support was added in #464. However, I recently found that calling ResultSet.getObject(int columnIndex) throws the following exception:

java.lang.RuntimeException: not implemented

	at org.apache.calcite.avatica.AvaticaSite.notImplemented(AvaticaSite.java:422)
	at org.apache.calcite.avatica.AvaticaSite.get(AvaticaSite.java:381)
	at org.apache.calcite.avatica.AvaticaResultSet.getObject(AvaticaResultSet.java:395)

The issue is that AvaticaSite does not implement support for it. I will put up a PR to add support by overriding our ResultSet.getObject method.