Action-Schema JSON validation

charm.ActionSpec now has a ValidateParams method which accepts an
unmarshaled JSON document, and validates it.  If it is valid, the
method returns true; otherwise, it returns false and a list of
errors returned by gojsonschema.

WIP, opened PR for comments
    // ValidateParams reports whether paramsMap, a JSON object
    // unmarshalled into an interface{}, conforms to the ActionSpec's Params.

?

(the "reports whether" phrasing is a standard Go idiom)