*: code preparations for supporting build range for like function on new collation columns | tidb-test=pr/2253

[time-and-fate]

What problem does this PR solve?

Issue Number: ref #48181

What changed and how does it work?

1. Some code formatting, variable renaming, and adding comments.
2. After we finish Can't use like to build range when new collation is enabled #48181, many values in ranges for string columns will become sort key (bytes) instead of the original value (usually string) like the previous. Because bytes and string have different printing formats when displaying the execution plan, this will make many changes in the displayed execution plan in the test cases. So I changed the formats of printing bytes to make it consistent with strings. (pkg/util/ranger/types.go)
3. For the fast path, setting the collation of a value to the collation of the column is moved from the executor builder into the optimizer. After we finish Can't use like to build range when new collation is enabled #48181, no matter what collation the column is, when it comes here, the datum might be the sort key, which must be bytes, and setting the datum to the collation of the column will be wrong. But this setting is needed, so we move it to an earlier stage. (pkg/executor/point_get.go, pkg/planner/core/point_get_plan.go)
4. Add test cases. So we can see the diff in the final PR.

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.

None

[codecov]

Codecov Report

Merging #48972 (791ce64) into master (b016589) will increase coverage by 1.3097%.
Report is 2 commits behind head on master.
The diff coverage is 95.1612%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #48972        +/-   ##
================================================
+ Coverage   70.9895%   72.2993%   +1.3097%     
================================================
  Files          1368       1397        +29     
  Lines        403893     412691      +8798     
================================================
+ Hits         286722     298373     +11651     
+ Misses        97205      95497      -1708     
+ Partials      19966      18821      -1145     

Flag	Coverage Δ
integration	43.6969% <95.1612%> (?)
unit	70.9874% <ø> (-0.0022%)	⬇️

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

Components	Coverage Δ
dumpling	53.9663% <ø> (ø)
parser	∅ <ø> (∅)
br	48.3541% <ø> (-4.6181%)	⬇️

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tangenta, winoros

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-11-29 10:41:21.436087348 +0000 UTC m=+1005710.101313545: ☑️ agreed by winoros.
• 2023-11-30 09:39:05.564220987 +0000 UTC m=+1088374.229447181: ☑️ agreed by tangenta.

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