feat: ✨ add grains (virtual actors) capabilities

[Tochemey]

closes #759

• Grains engine
• Unit tests
• Cleanup
• Examples
• cleanup

[codecov]

Codecov Report

Attention: Patch coverage is 83.82046% with 155 lines in your changes missing coverage. Please review.

Project coverage is 87.10%. Comparing base (a913f01) to head (13de5e6).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
actor/grain_factory.go	83.01%	39 Missing and 14 partials ⚠️
actor/grain_process.go	71.42%	35 Missing and 7 partials ⚠️
actor/grain_request.go	69.76%	13 Missing ⚠️
internal/cluster/engine.go	85.71%	9 Missing and 1 partial ⚠️
actor/actor_system.go	87.87%	5 Missing and 3 partials ⚠️
actor/rebalancer.go	89.85%	5 Missing and 2 partials ⚠️
actor/identity.go	84.61%	4 Missing and 2 partials ⚠️
actor/reflection.go	83.87%	2 Missing and 3 partials ⚠️
actor/grain_context.go	75.00%	4 Missing ⚠️
actor/grain_inbox.go	89.65%	2 Missing and 1 partial ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #760      +/-   ##
==========================================
- Coverage   87.56%   87.10%   -0.46%     
==========================================
  Files         102      111       +9     
  Lines        9095     9999     +904     
==========================================
+ Hits         7964     8710     +746     
- Misses        891     1014     +123     
- Partials      240      275      +35     

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