need support modify configuration online

It may also require the cooperation of tidb to complete

step by：

1. show config where name like '%apply-pool-size%';
2. set config tiflash raftstore-proxy.raftstore.apply-pool-size = 2;
3. have a error 1105：unknown type tiflash

expect to see：

1. Query OK, 0 rows affected, 3 warnings (0.09 sec)
2. show config where name like '%apply-pool-size%' and type = 'tilfash'; returns a modified value

version：
mysql> select tidb_version();
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version() |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v5.4.0
Edition: Community
Git Commit Hash: 55f3b24c1c9f506bd652ef1d162283541e428872
Git Branch: heads/refs/tags/v5.4.0
UTC Build Time: 2022-01-25 08:39:26
GoVersion: go1.16.4
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql>

Start by reproducing the three SQL steps against TiFlash with TiDB v5.4.0, focusing on the unknown type tiflash error from set config. Confirm that configuration changes can be applied online and that show config ... type = 'tiflash' reports the modified value without the error.