[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.

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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

• https://github.com/matrix-org/matrix-rust-sdk/pull/4140
• https://github.com/matrix-org/matrix-rust-sdk/pull/4149
• [ ] (nice to have) cache a thumbnail by media URI, discard the media thumbnail settings from the cache keys
• [ ] (nice to have) automatically retry to load a media if we failed to load it the first time in the timeline

Related

• https://github.com/matrix-org/matrix-rust-sdk/issues/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.