Implement "Tablet Targeting" RFC

[nickvanw]

Description

This PR adds the ability for a user to target a specific tablet by alias when doing USE, instead of only being able to pick a type for a shard. This means that a user can get "stable" routing, which is extremely useful when they don't have access to the underlying MySQL instance and need to do things like debug.

Related Issue(s)

#18808

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

AI Disclosure

This PR was written hand-in-hand with Claude Code, which absolutely got the initial few approaches extremely incorrect and required a lot of human help. Robot isn't going to take my job any time soon.

[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

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Documentation

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• New features should be documented.
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• There should be a comment at the top of each new or modified test to explain what the test does.

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Backward compatibility

• Protobuf changes should be wire-compatible.
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• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

❌ Patch coverage is 95.08197% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.69%. Comparing base (8fc596b) to head (6c49907).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/topo/topoproto/destination.go	95.65%	1 Missing ⚠️
go/vt/vtgate/executorcontext/vcursor_impl.go	92.30%	1 Missing ⚠️
go/vtbench/client.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #18809   +/-   ##
=======================================
  Coverage   69.68%   69.69%           
=======================================
  Files        1605     1606    +1     
  Lines      214461   214521   +60     
=======================================
+ Hits       149456   149518   +62     
+ Misses      65005    65003    -2     

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