Plugin framework to hook user provided scripts to run as part of CAT

[alexafshar]

Allow incorporating user provided scripts into CAT without touching main code base using plugings.

Two types of plugins:

• Standalone to run on its own without running CAT but still triggered using CAT CLI scripts.
• Integrated mode: run as part of the CAT run lifecycle as post-processing plugin that will get passed the context from the running job.
  provide demo plugins to demonstration

[alexafshar]

implemented in 1.8.x