refactor: rewrite the logic round-robin and least-load strategies

[Tochemey]

closes #973

1. Rewrite the implementation of round-robin and least-load placement/activation strategies for both actors and grains
2. Add a sample runnable code to test the grain round-robin activation

[codecov]

Codecov Report

❌ Patch coverage is 29.31034% with 123 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.93%. Comparing base (69cd561) to head (7d9082e).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
playground/issue-973/main.go	0.00%	123 Missing ⚠️

❌ Your patch check has failed because the patch coverage (29.31%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.
❌ Your project check has failed because the head coverage (86.93%) is below the target coverage (90.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #974      +/-   ##
==========================================
- Coverage   93.67%   86.93%   -6.75%     
==========================================
  Files         134      148      +14     
  Lines       10418    11266     +848     
==========================================
+ Hits         9759     9794      +35     
- Misses        425     1241     +816     
+ Partials      234      231       -3     

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