feat: ✨ add grains (virtual actors) capabilities

[Tochemey]

revised version of #760

[codecov]

Codecov Report

Attention: Patch coverage is 83.76561% with 169 lines in your changes missing coverage. Please review.

Project coverage is 87.10%. Comparing base (559c61b) to head (717861d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
actor/grain_engine.go	81.25%	37 Missing and 14 partials ⚠️
actor/actor_system.go	63.00%	32 Missing and 5 partials ⚠️
internal/cluster/engine.go	78.19%	23 Missing and 6 partials ⚠️
actor/rebalancer.go	81.95%	17 Missing and 7 partials ⚠️
actor/grain_pid.go	90.08%	8 Missing and 4 partials ⚠️
actor/grain_context.go	85.91%	10 Missing ⚠️
actor/grain_mailbox.go	89.28%	2 Missing and 1 partial ⚠️
internal/duration/duration.go	94.82%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #802      +/-   ##
==========================================
- Coverage   87.38%   87.10%   -0.28%     
==========================================
  Files         100      108       +8     
  Lines        9107    10076     +969     
==========================================
+ Hits         7958     8777     +819     
- Misses        906     1022     +116     
- Partials      243      277      +34     

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[sdil]

@Tochemey typo