Expose form validation methods for direct usage

[pikajude]

Discussed in #1914

^{Originally posted by pikajude September 21, 2021}
I can accept a Result<Form<T>, Errors<'_>> in a form handler which seems to do the validation automatically, but I can't find any manual way to apply the validation logic to an object I already have (i.e. from Json<T>)

I'm foregoing forms in Rocket entirely and writing a JSON API which my JS frontend will also use so I don't have to write two versions of every endpoint that my app has. However, I don't think I can use form validation on request data manually.

[pikajude]

Sorry, looks like this is a duplicate of #1915.