Add support for Custom API testing

[nillas12]

Is your feature request related to a problem? Please describe.
The Custom API as described here is a way to add custom integrations to CRM.
They are coded basically like workflows or plugins (business events).
https://docs.microsoft.com/en-us/power-apps/developer/data-platform/custom-api

Describe the solution you'd like
The ability to hook directly into the Custom API event with XRMMockup

Describe alternatives you've considered
Test against the business logic instead.

Additional context
Add any other context or screenshots about the feature request here.