Change healthcheck to only reload specific keyspace shard tablets

[GuptaManan100]

Description

This PR fixes the issue described in #17876. As the issue points out, we should only be loading tablets for the specific keyspace-shard that we don't have a primary for anymore.

This PR makes this change, by changing the load tablets trigger, to also store the keyspace shard information, and we use that to only load some of the tablets.

As part of this change, I also found that as part of the PR #17071 we introduced a couple of fields to hte GetTabletsByCellOptions but they weren't being respected in the GetTabletsMap function. This PR fixes this too, and adds tests for all the topo functions touched. The implementation of a couple of them has also been changed to improve efficiency.

Related Issue(s)

• Fixes Bug Report: Wasteful Topo calls in loadTabletsTrigger in the healthcheck #17876

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]

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[codecov]

Codecov Report

Attention: Patch coverage is 89.00000% with 11 lines in your changes missing coverage. Please review.

Project coverage is 67.53%. Comparing base (b05df12) to head (36bc2b6).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/topo/tablet.go	88.88%	6 Missing ⚠️
go/vt/discovery/topology_watcher.go	86.66%	4 Missing ⚠️
go/vt/topo/shard.go	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17872      +/-   ##
==========================================
+ Coverage   67.45%   67.53%   +0.07%     
==========================================
  Files        1594     1595       +1     
  Lines      259064   259701     +637     
==========================================
+ Hits       174760   175377     +617     
- Misses      84304    84324      +20     

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[deepthi]

The changes to this func make it correct, but slower. Previously the keyspace and shard were being ignored and we'd return all tablets. Where is this func being used, and what is the impact of fetching each tablet from topo versus (in the best case) a single topo call fetching all tablets?

[GuptaManan100]

It is being used only in VTOrc to refresh the tablets in a specific keyspace shard.