New features and bugfix

From #1:

• Add an option fdw_convert_tz to allow time zone conversion from UTC to a desired time zone (see docs/timezone)

From #2:

• Implement remote counting and grouping (docs/remote_grouping)

From #3:

• Implement BigQuery CAST() feature (see https://cloud.google.com/bigquery/docs/reference/standard-sql/functions-and-operators#casting and docs/casting)

Bugfix:

• _PARTITIONTIME pseudo column operator was hardcoded: commits/f979d13

Update bigquery_fdw with:

pip3 install --upgrade bigquery-fdw