executor: fix wrong behavior when ENUM column meet Aggregation (#14035)

[sre-bot]

cherry-pick #14035 to release-3.0

---

What problem does this PR solve?

Fix wrong behavior when ENUM column meet Aggregation as #13027

What is changed and how it works?

Add firstRow4Enum, the first_row executor for ENUM

Check List

Tests

• Unit test
• Manual test (add detailed scripts or steps below)

mysql> create table t(a enum('a', 'b'));
Query OK, 0 rows affected (0.01 sec)

mysql> insert into t values('a'), ('b');
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select cast(a as signed) from (select a from t group by a) t;
+-------------------+
| cast(a as signed) |
+-------------------+
|                 1 |
|                 2 |
+-------------------+
2 rows in set (0.00 sec)

mysql> 

[sre-bot]

/run-all-tests

[SunRunAway]

LGTM

[zz-jason]

LGTM

[sre-bot]

/run-all-tests