refactoring(plugins): plugins refactoring

[rustatian]

Reason for This PR

• Cyclic dependencies.
• PHP files mixed with Go.

Description of Changes

• Move all PHP files to the php_test_dir folder, update all plugins paths.
• Remove Availabler interface as leading to the cyclic dependencies.
• informer.List will return all plugins with the workers inside.

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]

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

[codecov[bot]]

Codecov Report

Merging #151 (e83579f) into master (f4a5fce) will increase coverage by 1.97%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #151      +/-   ##
==========================================
+ Coverage   65.29%   67.26%   +1.97%     
==========================================
  Files         139      135       -4     
  Lines       11032    10287     -745     
==========================================
- Hits         7203     6920     -283     
+ Misses       3096     2649     -447     
+ Partials      733      718      -15     

Impacted Files	Coverage Δ
amqp/plugin.go	100.00% <ø> (+9.09%)	:arrow_up:
beanstalk/plugin.go	100.00% <ø> (+6.66%)	:arrow_up:
boltdb/plugin.go	82.35% <ø> (+4.57%)	:arrow_up:
broadcast/plugin.go	72.41% <ø> (ø)
broadcast/rpc.go	69.81% <ø> (ø)
config/plugin.go	68.31% <ø> (-1.30%)	:arrow_down:
grpc/plugin.go	61.66% <ø> (+0.50%)	:arrow_up:
http/middleware/gzip/plugin.go	100.00% <ø> (+11.11%)	:arrow_up:
http/middleware/headers/plugin.go	87.83% <ø> (+1.17%)	:arrow_up:
http/middleware/new_relic/plugin.go	0.00% <ø> (ø)
... and 40 more

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