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)

• Fixes: RFC: Tablet Targeting #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

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[codecov]

Codecov Report

❌ Patch coverage is 93.84615% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.81%. Comparing base (d411dd0) to head (a637fc9).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/executorcontext/vcursor_impl.go	83.33%	2 Missing ⚠️
go/vt/topo/topoproto/destination.go	95.65%	1 Missing ⚠️
go/vtbench/client.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18809      +/-   ##
==========================================
- Coverage   69.82%   69.81%   -0.01%     
==========================================
  Files        1610     1610              
  Lines      215303   215391      +88     
==========================================
+ Hits       150328   150376      +48     
- Misses      64975    65015      +40     

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[nickvanw]

I believe this is ready for another review @harshit-gangal! Let me know if you have any questions.

[timvaillancourt]

@nickvanw let's say I use this feature to write rows to a PRIMARY that doesn't align with the VSchema, am I creating any data correctness/etc risks we should detail here?

I'm wondering if that would cause query failures or potentially-surprising results. But at the same time, I guess you can only use this feature with intention 🤔

And on a similar track, could vReplication get upset about rows in the wrong shard? One dreamed-up scenario I can think of is merging shards, and the same primary key exists in both, etc

None of what I'm thinking means changing the code here I think (it looks good so far), maybe just documentation