connection pool: max idle connections implementation

[harshit-gangal]

Description

This PR adds support for maximum idle connection in the pool.
The 3 pools gets it's own parameter for configuration:

• Non-Streaming: queryserver-config-query-pool-max-idle-count
• Streaming: queryserver-config-stream-pool-max-idle-count
• Transaction: queryserver-config-txpool-max-idle-count

Related Issue(s)

• Closes Feature Request: Add support for maximum idle connection in the pool #17471

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]

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[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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