send queue: allow editing/aborting/unwedging a media upload

[bnjbvr]

Followup of #1732:

• cancel an upload.
  • this needs to interrupt an upload, if it's still ongoing.
  • If something was uploaded, do we try to remove the uploaded content?
    • most likely case, we're aborting while uploading the content: if the thumbnail only was uploaded, it's easy to remove it
    • quite unlikely to happen in the real world: we're sending the media event, the content has been uploaded already; we could remove it.
    • more likely: remove after the media have been uploaded and event has been sent; in that case we'll only remove the event, as we do for any media events
      • we have no proofs the media hasn't been shared somewhere else, by us or somebody else (aka it's impossible to GC shareable content in a decentralized e2ee system?)
• edit caption / formatted_caption / filename
  • Should we attempt to clarify in which case it's a filename vs a caption?
• allow unwedging: send queue: implement full unwedging of media uploads #4258
  • if the upload fails non-intermittently (aka it's not network), it will return a matrix_sdk::Error at the send queue level, and a generic wedge error at the timeline level.
  • refactor!(send queue): move RoomSendQueue::unwedge to the SendHandle type #4226