feature: after_init option

[rustatian]

Reason for This PR

closes: roadrunner-server/roadrunner#1554

Description of Changes

• Command to be executed after pool initialization (number of executions equal to the number of plugins using the pool of workers).

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 #73 (2ea8e38) into master (1782417) will increase coverage by 0.31%.
The diff coverage is 100.00%.

❗ Current head 2ea8e38 differs from pull request most recent head b2bbcfa. Consider uploading reports for the commit b2bbcfa to get more accurate results

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
+ Coverage   82.56%   82.87%   +0.31%     
==========================================
  Files          22       22              
  Lines        1904     1904              
==========================================
+ Hits         1572     1578       +6     
+ Misses        277      271       -6     
  Partials       55       55              

Impacted Files	Coverage Δ
pool/config.go	100.00% <ø> (ø)
pool/static_pool/static_pool.go	71.98% <100.00%> (ø)

... and 1 file with indirect coverage changes