feat(send queue): send attachments with the send queue

[bnjbvr]

Fixes #1732. This adds support for sending medias via the send queue, which gives us local echoes for free.

The process looks like this:

• put the medias (thumbnail and file) into the media cache (with temporary MXC IDs)
• send a local echo for the media event that references the local media cache
• if thumbnail
  • create a request to upload the thumbnail
  • create a dependent request to upload the file, depending on the thumbnail being uploaded
  • create a dependent request to send the media event, depending on the file upload
• otherwise
  • create a request to upload the file
  • create a dependent request to send the media event
• once the medias have been uploaded, send a local echo edit for the media event, that fixes the MXC IDs to the final ones

The send queue dependency system is made such that, a dependent request depends on at most one other request. The design chosen in this PR is that dependencies are kept linear, each request depends on the previous one to be finished. As such, it means that when uploading a thumbnail, we need to remember the thumbnail MXC ID when uploading the file, because the file request will pass that information (when it's completed) to the media event.

---

This can not be reviewed commit by commit. Instead, I'd recommend looking at the commit which improves the module doc comment first: 1d0ba02

And then review the rest of the changes; we can also have a call to do the review together.

Sits on top of #4199 and #4200.

[codecov]

Codecov Report

Attention: Patch coverage is 78.32370% with 75 lines in your changes missing coverage. Please review.

Project coverage is 84.82%. Comparing base (0942dab) to head (d1c34b5).
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/send_queue.rs	79.14%	34 Missing ⚠️
crates/matrix-sdk/src/send_queue/upload.rs	81.63%	27 Missing ⚠️
crates/matrix-sdk-ui/src/timeline/futures.rs	0.00%	10 Missing ⚠️
...rates/matrix-sdk-ui/src/timeline/controller/mod.rs	0.00%	2 Missing ⚠️
crates/matrix-sdk/src/media.rs	88.88%	1 Missing ⚠️
crates/matrix-sdk/src/room/mod.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4195      +/-   ##
==========================================
- Coverage   84.87%   84.82%   -0.06%     
==========================================
  Files         272      273       +1     
  Lines       29251    29519     +268     
==========================================
+ Hits        24828    25040     +212     
- Misses       4423     4479      +56     

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[Hywan]

Did a pair-review via Element Call. I'm approving this PR, it's an excellent code. @bnjbvr has taken notes for things that are left to do, but nothing blocking the merge.

Impressive PR. Well done!