Provide a fix control switch to fix overestimation of index scan of inner side of index join

[time-and-fate]

Background

In some cases, tidb will produce an extremely high estimation for the IndexRangeScan under the inner side of an IndexJoin. In #41996 we resolved this.

But we reverted this change in #42362 because in an internal workload, it caused execution plan regression. As explained in #42362, it's not because something incorrect in #41996, but better estimation for one operator doesn't guarantee a better execution plan as a whole.

However, this improvement is important in some scenarios, and we want to make users able to use it when it's necessary. So we plan to add this to tidb_opt_fix_control.

Usage

-- session level
set @@tidb_opt_fix_control = '44855:ON';
-- global level
set global tidb_opt_fix_control = '44855:ON';