planner: add maybe good heuristics for index selection

[xuyifangreeneyes]

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary:

What is changed and how it works?

When SQL has order by pk limit n pattern, and one index path only has point ranges and its CountAfterAccess is small enough, we directly choose the index rather than pk. Since the heuristic rule is not always good, we introduce a global/session variable tidb_enable_maybe_good_heuristics and only apply the rule when the switch turns on. When executing EXPLAIN, TiDB will output a NOTE level warning if some path matches the maybe good heuristic rule.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

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

introduce maybe good heuristics for index selection

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[xuyifangreeneyes]

/sig planner

[xuyifangreeneyes]

/cc @winoros

[xuyifangreeneyes]

Since #27123 can also make the optimizer choose index rather than pk for the situations like where index_col = ... order by pk limit n, the pr can be closed.