Add type mapping for 'pg_sys::TimestampTz' to 'timestamp with time zone'

This allows the `pg_sys::TimestampTz' type to be used in function
signatures, providing zero-cost access to the underlying i64 timestamp
of a 'timestamp with time zone' field.

pgx/src/lib.rs

@@ -143,6 +143,7 @@ pub static DEFAULT_SOURCE_ONLY_SQL_MAPPING: Lazy<HashSet<RustSourceOnlySqlMappin
         let mut m = HashSet::new();
 
         map_source_only!(m, pg_sys::Oid, "Oid");
+        map_source_only!(m, pg_sys::TimestampTz, "timestamp with time zone");
 
         m
     });