Support MSC4016's resumable uploads component

[Travis Ralston]

https://developers.google.com/youtube/v3/guides/using_resumable_upload_protocol#Resume_Upload
matrix-org/matrix-spec-proposals#4016

[Matthew Hodgson]

n.b. this has two components:

• support streaming transfers (ie let users download files which wre still uploading) - which is arguably the most useful bit
• separately, support resumable uploads via Content-Range headers on PUT

[Travis Ralston]

I believe MMR already implicitly supports the first, but doesn't support the second. Would largely depend on how the stop signal is sent.