Integrate jsonschema into ``Payload`` (or subclass?)

[harlowja]

Typically if u have a raw payload that is a known json blob/payload it is very much desireable to be able to ensure that payload matches a known schema before more field reading/consumption occurs (aka, right after a payload gets read, validate it as being valid/invalid and throw a matching exception); naturally for json blobs https://python-jsonschema.readthedocs.org/en/latest/ makes this really easy so it might be nicer to have tighter integration natively built-in to suitecase?)