feature: Custom Validators (webhooks)

[stanistan]

Currently, cmmc only supports doing JSONSchema validation.

It would be nice to support something like a custom validators via some syntax in the MergeTarget like:

   # ....
   data: 
     key:
       validator:
         url: <...>

For this to be usable, this should also have a proto definition of a request/response spec of a sort.