planner, expr: eval readonly user var during plan phase

[winoros]

What problem does this PR solve?

Issue Number: close #52742, #8733, #14231, #24276

Problem Summary:

What changed and how does it work?

If a user var is not affected by setvar at SQL level, we can directly use its value. No need to wrap it with getvar.

Check List

Tests

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

  • I checked and no code files have been changed.

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

Please refer to Release Notes Language Style Guide to write a quality release note.

Support fold read-only user variables to constants
支持将只读的用户变量折叠为常量

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 22 lines in your changes missing coverage. Please review.

Project coverage is 56.0074%. Comparing base (ef0a3eb) to head (c8dc5cb).

Additional details and impacted files

@@                Coverage Diff                @@
##             master     #54462         +/-   ##
=================================================
- Coverage   72.8631%   56.0074%   -16.8557%     
=================================================
  Files          1544       1665        +121     
  Lines        436218     608423     +172205     
=================================================
+ Hits         317842     340762      +22920     
- Misses        98816     244344     +145528     
- Partials      19560      23317       +3757     

Flag	Coverage Δ
integration	37.0928% <64.9350%> (?)
unit	71.8237% <71.4285%> (-0.0653%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	52.9656% <ø> (ø)
parser	∅ <ø> (∅)
br	52.5774% <ø> (+6.7065%)	⬆️

[ti-chi-bot]

@winoros: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/mysql-test	c8dc5cb	link	true	/test mysql-test

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