Support API modifier

[poscat0x04]

It would be nice if we can distinguish between optional and required arguments.

[alpmestan]

Are you talking about individual components of the "form" being optional/required, or the whole multipart request body?

[poscat0x04]

Optional individual components, that is to distinguish optional/required/multiple at the type level, much like QueryParam' and QueryParams.

[alpmestan]

Ah... Hmm, well it's not entirely trivial to implement, you'd quite likely have to write your own variant on the multipart combinator from this package.

You can already have optional/required "fields" in the form, merely by using a Maybe for the optional fields, but the design of this package is such that you need to parse all the optional/required values in a single "aggregated" data type. Separating everything is a lot trickier I think.