feature: dynamic workers scaling

[rustatian]

Reason for This PR

closes: roadrunner-server/roadrunner#1728

Description of Changes

• Add two new methods to allocate and de-allocate a PHP worker.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the MIT license.

PR Checklist

[Author TODO: Meet these criteria.]
[Reviewer TODO: Verify that these criteria are met. Request changes if not]

• All commits in this PR are signed (git commit -s).
• The reason for this PR is clearly provided (issue no. or explanation).
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this PR.
• Any user-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.

[codecov]

Codecov Report

Merging #95 (fe59f30) into master (02b0591) will decrease coverage by 0.03%.
Report is 3 commits behind head on master.
The diff coverage is 82.45%.

@@            Coverage Diff             @@
##           master      #95      +/-   ##
==========================================
- Coverage   77.61%   77.58%   -0.03%     
==========================================
  Files          23       23              
  Lines        2064     2066       +2     
==========================================
+ Hits         1602     1603       +1     
  Misses        410      410              
- Partials       52       53       +1     

Files Changed	Coverage Δ
worker/worker.go	67.85% <62.50%> (-0.19%)	⬇️
worker_watcher/container/channel/vec.go	78.48% <68.00%> (+1.71%)	⬆️
worker_watcher/worker_watcher.go	85.90% <88.29%> (-3.21%)	⬇️
fsm/fsm.go	69.04% <100.00%> (+0.75%)	⬆️
pool/static_pool/supervisor.go	96.39% <100.00%> (+2.81%)	⬆️
pool/static_pool/workers_pool.go	70.74% <100.00%> (+0.92%)	⬆️

... and 1 file with indirect coverage changes