feat: normalize TIMESTAMPTZ values to UTC in PgSQL remote executor

PostgreSQL returns TIMESTAMPTZ column values as text like
"2024-06-15 14:30:00+05:30" where the trailing offset was previously
ignored (timezone handling was explicitly deferred). Switch the
TIMESTAMPTZOID branch to parse_datetime_tz() so the offset is applied
and the stored microseconds-since-epoch value is normalized to UTC.

Author: renecannao

src/sql_engine/pgsql_remote_executor.cpp

@@ -218,9 +218,8 @@ Value PgSQLRemoteExecutor::pg_field_to_value(
         }
 
         case TIMESTAMPTZOID: {
-            // PostgreSQL returns "YYYY-MM-DD HH:MM:SS+TZ" -- parse the datetime
-            // part, ignoring timezone for now (as per spec: timezone handling deferred).
-            int64_t us = datetime_parse::parse_datetime(data);
+            // PostgreSQL returns "YYYY-MM-DD HH:MM:SS+TZ". Parse and normalize to UTC.
+            int64_t us = datetime_parse::parse_datetime_tz(data);
             return value_timestamp(us);
         }