DNM: planner: cherry-pick enforce-mpp to v5.0

[LittleFall]

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary:

5.0 needs enforce-mpp.

What is changed and how it works?

cherry-pick these prs to 5.0:

1. planner: support explain verbose mode #23926 verbose explain
2. planner: let CopTiFlashConcurrencyFactor inflence the cost of whole plan #24157 CopTiFlashConcurrencyFactor
3. [reverted]planner: support set tidb_allow_mpp to 2 or ENFORCE to enforce use mpp mode. #24516 support allow_mpp=2
4. planner: Revert tidb_allow_mpp modification for downgrade compatibility and add warnings for enforce mpp. #25302 revert 24516 and support tidb_enforce_mpp and add warning
5. planner: enforce projection push down #25450 enforce projection push down
6. planner: Log warnings when agg function can not be pushdown in explain statement #25553 fix warning
7. planner: add warnings about joins when tidb_enforce_mpp is enabled. #26246 add warning o fjoin

Check List

Tests

• Unit test
• Integration test

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

• Support set tidb_enforce_mpp=1 to enforce use mpp mode.

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[ti-chi-bot]

@LittleFall: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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[LittleFall]

committed by #26382