Add support for showing global gtid executed per shard

[harshit-gangal]

Description

This PR adds a new query to show gtid from all shards from a keyspace.

mysql> show global gtid_executed;
+----------+-------------------------------------------+-------+
| db_name  | gtid_executed                             | shard |
+----------+-------------------------------------------+-------+
| commerce | bfb145e3-98ec-11eb-b9a7-98af65a6dc4a:1-23 | 0     |
+----------+-------------------------------------------+-------+
1 row in set (0.00 sec)

Related Issue(s)

• resolves feature request: VTGate support for retrieving GTIDs from shards #7400

Checklist

• Tests were added or are not required
• Documentation was added or is not required

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving

[systay]

Clever to use order aggregate like that. ⭐