[Media+Timeline] Local echo support for media attachments

[stefanceriu]

Sending media attachments through the Timeline should make use of local echoes for a seamless experience.

---

Sub project: handle media upload in the send queue

• break apart uploading in a small state machine that can be reused in the send queue
• modify the send queue storage format so it takes care of "requests", not only sending "event content"
• add a new request type for uploading a media
• (draw the rest of the freaking howl)
• handle medias local echoes in the timeline
• allow cancelling a media upload (be careful about the state of the upload machine, consider whether it's worth deleting uploaded content or not)

Tasks

• refactor(media): some preparatory refactorings #4140
• media: cache media's content after uploading it #4149 (cache media and thumbnail)
• (nice to have) automatically retry to load a media if we failed to load it the first time in the timeline

Related

• [Media+Timeline] Expose media uploading progress through the timeline items #1968
• element-meta task

[jplatte]

Synapse implements asyncronous uploads from v1.97.0 onwards, with which this should be easier to implement (can generate a transaction ID / get an event ID for it in parallel with the upload). Ruma also supports the new endpoint.