Add/Remove handlers without code changes

[dluc]

Motivation and Context (Why the change? What's the scenario?)

The current KM Service definition requires editing and rebuilding code/images in order to add/remove pipeline handlers.
This PR allows to add/remove handlers simply by editing the configuration files, providing the required assembly without the need to rebuild.

High level description (Approach, Design)

• Remove from KM Builder the hardcoded list of handlers used for the service. The list remains only for the serverless option.
• Add to service configuration the list of handlers to load.
• Us reflection to load handlers from assemblies.