feat: round-robin selector

[waynexia]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

What's changed and what's your intention?

Implement RoundRobin Selector which selects peers in a completely fair way.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.

[codecov]

Codecov Report

Attention: Patch coverage is 96.94656% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 85.16%. Comparing base (4778575) to head (8601552).
Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4024      +/-   ##
==========================================
- Coverage   85.50%   85.16%   -0.34%     
==========================================
  Files         977      981       +4     
  Lines      169344   170256     +912     
==========================================
+ Hits       144793   144998     +205     
- Misses      24551    25258     +707