Add session algorithm to tablet balancer

[mhamza15]

Description

This PR is a continuation of #18552, since I no longer have access to the old fork 😅. It's been slightly refactored to match the changes introduced in #18787.

This PR introduces a "session" tablet balancer, which will direct a session to the same tablet for the duration of the session. This protects against cases where a session reads a value from one tablet, and then gets routed to a different, higher-lag tablet and reads an older value.

The session balancer initially used consistent hashing, but was later switched to use rendezvous hashing for simplicity. The tradeoff there is that rendezvous hashing has an O(n) runtime, n being the number of tablets per target (keyspace + shard + tablet type + cell). As the number of tablets per target is expected to be small, the performance should be reasonable (I plan on benchmarking this).

Related Issue(s)

Closes #11971

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

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[FINDMEAI]

nice bro

[codecov]

Codecov Report

❌ Patch coverage is 81.48148% with 25 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@3dd1516). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
go/vt/vttablet/queryservice/wrapped.go	73.33%	16 Missing ⚠️
go/vt/vtgate/balancer/balancer.go	50.00%	4 Missing ⚠️
go/vt/vtgate/balancer/session.go	91.17%	3 Missing ⚠️
go/vt/vtgate/tabletgateway.go	92.85%	2 Missing ⚠️

Additional details and impacted files

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  Coverage        ?   69.82%           
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  Files           ?     1611           
  Lines           ?   215427           
  Branches        ?        0           
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  Hits            ?   150416           
  Misses          ?    65011           
  Partials        ?        0           

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