Projection pushdown to tikv feature GA

## Enhancement
tidb_opt_projection_push_down is introduced by #27029 , and is set 'off' by default, still not GA now.
It aimed to improve performance in two sides:

1. Utilize distributed tikv nodes to perform calculations parallelly
2. Provide an elegantly way to reduce unused columns which are transformed back to tidb from tikv #44902

However, there are still some basic functionality issues when this feature is set 'on':
- [x] #34993
- [x] #51874

Also exposed some existing function behavior inconsistency issues:
- [x] #51877
- [x] #51879
- [x] #51880
- [x] #51881
- [ ] #51882

And after these functionality issues are fixed, we need to consider the performance impact

- [x] #53613

Others:

- [x] #53831
- [x] https://github.com/PingCAP-QE/tidb-test/pull/2348
- [x] https://github.com/pingcap/docs-cn/pull/17891