Projection pushdown to tikv feature GA

[yibin87]

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 Is it possibe to support Projection push down on IndexLookUp? #44902

However, there are still some basic functionality issues when this feature is set 'on':

• Projection push down make tikv crashed. #34993
• Projection push down to tikv leads to incorrect result when subquery and agg #51874

Also exposed some existing function behavior inconsistency issues:

• Tikv conv function inconsistent results with tidb #51877
• Tikv cast string as decimal function inconsistent behavior with tidb #51879
• Tikv lead function inconsistent behavior with tidb #51880
• Tikv several time/date realted function inconsistent behavior with tidb #51881
• Tikv inconsistent warning code and message with tidb #51882

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

• Limit projection pushed down scope to avoid potential performance degression #53613

Others:

• config: Change tidb_opt_projection_push_down default value to true | tidb-test=pr/2359 #53831
• https://github.com/PingCAP-QE/tidb-test/pull/2348
• Change tidb_opt_projection_push_down default value to ON docs-cn#17891

[yibin87]

/assign @yibin87