Resumable Uploads: `application/partial-upload` media type

[Acconut]

As discussed in https://github.com/httpwg/http-extensions/issues/2501 and https://github.com/httpwg/http-extensions/issues/2610, the PATCH requests needs a suiting media type to describe the message content.

Since there does not seem to be an existing, suitable media type, we proposed the application/partial-upload media type on the media-types mailing list. This PR is a first, rough attempt at including such a media type in the draft.