feat: add plugin system to workit

[OlivierAlbertini]

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closes #217

• update docs Add documentation for plugin system #218
• add example (in examples folder) Add documentation for plugin system #218

[codecov-io]

Codecov Report

Merging #216 (d75fcc8) into master (55b08bd) will increase coverage by 2.86%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #216      +/-   ##
==========================================
+ Coverage   70.22%   73.08%   +2.86%     
==========================================
  Files          49       50       +1     
  Lines        1041     1111      +70     
  Branches      133      142       +9     
==========================================
+ Hits          731      812      +81     
+ Misses        310      299      -11     

Impacted Files	Coverage Δ
packages/workit-bpm-client/src/camundaBpmClient.ts	96.00% <75.00%> (-2.51%)	⬇️
packages/workit-zeebe-client/src/zeebeClient.ts	71.42% <75.00%> (+0.34%)	⬆️
packages/workit-core/src/plugin/pluginLoader.ts	88.46% <88.46%> (ø)
packages/workit-core/src/config/container.ts	95.45% <100.00%> (+0.45%)	⬆️
packages/workit-core/src/IoC.ts	34.06% <0.00%> (+23.07%)	⬆️

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