feat: expose an API for custom mailboxes

[Tochemey]

closes #478

• Expose mailbox API
• Implement UnboundedMailbox using mpsc queue which is the default mailbox
• Implement BoundedMailbox using the ring buffer queue
• Add a SpawnOption to spawn actor with custom mailbox if needed
• Add some performance enhancement
• Add benchmark code for both UnboundedMailbox and BoundedMailbox

[codecov]

Codecov Report

Attention: Patch coverage is 98.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 88.26%. Comparing base (9343db4) to head (d0a48b5).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
actors/pid.go	91.66%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #479      +/-   ##
==========================================
+ Coverage   88.03%   88.26%   +0.22%     
==========================================
  Files          43       45       +2     
  Lines        3444     3518      +74     
==========================================
+ Hits         3032     3105      +73     
- Misses        293      294       +1     
  Partials      119      119              

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