Add sticky choice option for leastping

[xiaokangwang]

(AI Generated Summary)
This pull request adds a new "sticky choice" feature to the StrategyLeastPingConfig load balancing strategy. This feature allows the strategy to remember and reuse the last selected outbound when enabled, potentially improving connection stability. The implementation introduces a new configuration field, updates the protobuf definitions and generated code, and modifies the strategy logic to support this behavior.

Sticky choice feature for least ping strategy:

• Added a new sticky_choice boolean field to the StrategyLeastPingConfig protobuf message and regenerated the corresponding Go code to include this option. [1] [2] [3] [4]
• Updated the LeastPingStrategy struct in strategy_leastping.go to include a mutex and a lastChoice field for thread-safe tracking of the last selected outbound. [1] [2]
• Modified the PickOutbound method to reuse the last selected outbound if sticky_choice is enabled and no new outbound is selected, ensuring thread safety with locking.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 23.17%. Comparing base (7fdb717) to head (abff2b3).
⚠️ Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
app/router/strategy_leastping.go	0.00%	5 Missing ⚠️
app/router/config.pb.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3621      +/-   ##
==========================================
- Coverage   23.21%   23.17%   -0.04%     
==========================================
  Files         836      836              
  Lines       45454    45466      +12     
==========================================
- Hits        10552    10539      -13     
- Misses      33685    33707      +22     
- Partials     1217     1220       +3     

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