Fine grained privileges to use `set tiflash replica <n>`

Feature Request

Is your feature request related to a problem? Please describe:
alter statement like alter table <table_name> set tiflash replica <n> DDL creates TiFlash replica.
Then the privileges to create/delete replica is alter.

I wish TiFlash has a file grained privileges to use set tiflash replica <n>. Like users can run 'alter table <table_name> add column' while it is prohibited to 'set tiflash replica '.

Describe the feature you'd like:
Create a dedicated privileges to allow/disallow users to run alter table <table_name> set tiflash replica <n>

Here are steps to represent this request:

1. Startup tiup playground with --tiflash 1 option

tiup playground --tiflash 1

2. Open another terminal to connect to the tiup playground instance as root.

mysql --comments --host 127.0.0.1 --port 4000 -u root

3. As a root user, create test1 database.

create database test1;

3. As a root user, create user1 and user2 users.
   The user1 has all privileges on all of test1 database. user2 has only alter privileges on test1 database.

create user 'user1'@'localhost';
create user 'user2'@'localhost';
grant all privileges on test1.* to 'user1'@'localhost';
grant alter on test1.* to 'user2'@'localhost';

4. Exit the root connection

5. Connect to tiup playground instance as user1

mysql --comments --host 127.0.0.1 --port 4000 -u user1;

6. Use test1 database and create t1 table

use test1;
create table t1 (id int not null auto_increment, primary key(id));
insert into t1 values();

7. Exit the user1 connection

8. Connect to tiup playground instance as user2

mysql --comments --host 127.0.0.1 --port 4000 -u user2;

9. Execute alter table test1.t1 add column

mysql> alter table test1.t1 add var varchar(10);
Query OK, 0 rows affected (0.69 sec)

10. Execute alter table test1.t1 set tiflash replica <n>

mysql> alter table test1.t1 set tiflash replica 1;
Query OK, 0 rows affected (0.39 sec)

• Expected behavior: Execute alter table test1.t1 set tiflash replica <n> should be prohibited for user2.
• Actual behavior: Execute alter table test1.t1 set tiflash replica <n> is allowed for user2.

Describe alternatives you've considered:
No alternative solution found so far

Teachability, Documentation, Adoption, Migration Strategy:

Start by reproducing the privilege behavior with the documented tiup playground --tiflash 1 setup and the user1/user2 SQL commands. No source files or tests are named in the issue, so the privilege-checking path for ALTER TABLE must be located first. Done means user2 can still use ordinary ALTER operations but is prohibited from SET TIFLASH REPLICA.