connection pool: max idle connections implementation

[harshit-gangal]

Description

Related Issue(s)

Checklist

• 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:

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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 73.33333% with 8 lines in your changes missing coverage. Please review.

Project coverage is 67.66%. Comparing base (9383943) to head (10025af).

Files with missing lines	Patch %	Lines
go/pools/smartconnpool/pool.go	73.33%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17443      +/-   ##
==========================================
- Coverage   67.67%   67.66%   -0.01%     
==========================================
  Files        1583     1583              
  Lines      254363   254394      +31     
==========================================
- Hits       172140   172137       -3     
- Misses      82223    82257      +34     

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