Support Compaction via SQL command

[breezewish]

Enhancement

Recently the PageStorage v3 engine was introduced to TiFlash. By allowing users to manually trigger the compaction, the migration from PageStorage v2 to PageStorage v3 can be done easily, as a compaction command will merge and clear all delta layer data (which was stored in v2) into the stable layer, while new delta layer data will be stored in PageStorage v3.
As a bonus, even when the Delta layer is already stored in PageStorage v3, users can also benefit from this manual compaction command, considering that compaction will rewrite the stored data into a better organized state.

Consider providing SQL which can execute directly from TiDB:

alter table T compact tiflash replica; 

Related TiFlash issue: pingcap/tiflash#4145 pingcap/tiflash#4897

[breezewish]

Implemented in #34741