Enhancement: Document and Demonstrate via example, making extension / re-use of a existing scheme (like PSA)

[yogeshbdeshpande]

Veraison implementation is using standard schemes like PSA/DICE and demonstrating this via README.md for usage.

This feature enhancement discusses ways and means to explain how:

A specific scheme like PSA can be extended by a Vendor specific Profile of PSA to add vendor specific claims.
Introduce helpful documentation with implementing Vendor specific Plugins.
Anything else required to complete end to end implemation (example enhancing CoRIM/CoMID Encoder Decoder and so is CoCli)

[yogeshbdeshpande]

This issue will cover the documentation for the feature:
For code changes required for demonstration see:
veraison/veraison#154