Support promotion rule override in vttablet

[timvaillancourt]

Description

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• Apply the release notes (needs details) label if users need to know about this change.
• New features should be documented.
• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

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If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow needs to be marked as required, the maintainer team must be notified.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

Attention: Patch coverage is 5.55556% with 34 lines in your changes missing coverage. Please review.

Project coverage is 68.70%. Comparing base (7a737f4) to head (31c3627).
Report is 37 commits behind head on main.

Files	Patch %	Lines
...vtctl/reparentutil/promotionrule/promotion_rule.go	0.00%	14 Missing ⚠️
go/vt/topo/topoproto/flag.go	0.00%	10 Missing ⚠️
go/vt/topo/topoproto/tablet.go	0.00%	5 Missing ⚠️
go/vt/vtctl/reparentutil/durability.go	20.00%	4 Missing ⚠️
go/vt/vttablet/tabletmanager/tm_init.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16343      +/-   ##
==========================================
+ Coverage   68.61%   68.70%   +0.09%     
==========================================
  Files        1544     1547       +3     
  Lines      197993   198355     +362     
==========================================
+ Hits       135848   136280     +432     
+ Misses      62145    62075      -70     

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