Limit the concurrent number of sst files that call ingest sst API during the `add index` period

## Enhancement
related to https://github.com/pingcap/tidb/issues/58807 

currently, we already have https://docs.pingcap.com/tidb/stable/system-variables/#tidb_ddl_reorg_max_write_speed-new-in-v6512-v755-and-v850 
while we can not control the concurrent number of sst files, and TiKV will take the number of level0 files as the standard for rate limiting