Add :form and :multipart formats

[cllns]

These are commonly used for web apps so we should have first-class support for them.

Related: #439

Also, I made Hanami::Action::Mime::TYPES into @api public (from private). The values are already used in apps so it's effectively already public API, so we might as well show them in the API docs. We already link to the constant in the guides, so in the future that could be a link to API docs (once those are working properly).