Add support for IETF resumable upload draft

[Acconut]

This patch provides a first, rough support for the new, upcoming resumable uploads draft. Details of its specification are currently available at https://datatracker.ietf.org/doc/draft-ietf-httpbis-resumable-upload/01/. With this PR we can gain first experience about the aspects and issues with implementing the draft.

This work is not yet ready for production. As details of the specification change, this PR will change as well.