Plugin Circuit Breaker

[italolelis]

What does this PR do?

• Adds the CB plugin
• It is based on hystrix from Netflix
• It uses hystrix-go
• It contains examples with a hystrix-dashboard
• Changes the plugin signature to pass the API definition

Closes #301

[codecov[bot]]

Codecov Report

Merging #309 into master will increase coverage by 1.71%. The diff coverage is 68.78%.

@@            Coverage Diff             @@
##           master     #309      +/-   ##
==========================================
+ Coverage   39.04%   40.75%   +1.71%     
==========================================
  Files          68       71       +3     
  Lines        2359     2505     +146     
==========================================
+ Hits          921     1021     +100     
- Misses       1381     1422      +41     
- Partials       57       62       +5

Impacted Files	Coverage Δ
pkg/config/specification.go	59.61% <0%> (ø)	:arrow_up:
pkg/plugin/oauth2/setup.go	7.86% <0%> (ø)	:arrow_up:
pkg/plugin/requesttransformer/setup.go	81.81% <100%> (ø)	:arrow_up:
pkg/plugin/basic/setup.go	83.78% <100%> (ø)	:arrow_up:
pkg/plugin/compression/setup.go	100% <100%> (ø)	:arrow_up:
pkg/plugin/rate/setup.go	57.77% <100%> (ø)	:arrow_up:
pkg/plugin/cors/setup.go	88.88% <100%> (ø)	:arrow_up:
pkg/plugin/bodylmt/setup.go	81.81% <100%> (ø)	:arrow_up:
pkg/plugin/retry/setup.go	39.13% <100%> (ø)	:arrow_up:
pkg/plugin/responsetransformer/setup.go	81.81% <100%> (ø)	:arrow_up:
... and 6 more

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