create SCEP validationMethodCustom

[mishaslavin]

Name of feature: Custom Challenge validator API

Pain or issue this feature alleviates:

There are situations where SCEP challenges could be validated using a backend library or some other service, rather than a webhook or static challenge. This creates an interface where such a validation could occur

Why is this important to the project (if not answered above):

Allows creation of new types of challenge validation / authorization integrations going forward

Is there documentation on how to use this feature? If so, where?

In what environments or workflows is this feature supported?

In what environments or workflows is this feature explicitly NOT supported (if any)?

Supporting links/other PRs/issues:

💔Thank you!