Produce query warnings for using features not supported when sharded

[systay]

Description

When a user is using Vitess in unsharded mode, but they know that they will need to shard to scale, enabling this feature will start producing query warnings when a feature is used that is not supported in sharded mode.
By adding the flag -warn_sharded_only=true to the vtgate start parameters, warnings will be produced for these features.

Example:

mysql> call sp_insert();
Query OK, 1 row affected, 1 warning (0.04 sec)

mysql> show warnings;
+---------+------+---------------------------------------------------------------+
| Level   | Code | Message                                                       |
+---------+------+---------------------------------------------------------------+
| Warning | 1235 | use of feature that is only supported in unsharded mode: CALL |
+---------+------+---------------------------------------------------------------+
1 row in set, 1 warning (0.00 sec)

Checklist

• Tests were added or are not required

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving