feat(composer): add support for attachments in drafts

[Johennes]

This adds support for storing attachments with composer drafts. The API allows for multiple attachments in the draft. I believe this is ok, because the SDK doesn't provide more functionality than saving and loading the draft. It is up to the consuming client to decide how to use it. It may limit its usage to a single attachment, send multiple attachments as individual events or use MSC4274 galleries.

• Public API changes documented in changelogs (optional)

[codspeed-hq]

CodSpeed Performance Report

Merging #5794 will not alter performance

_{Comparing Johennes:johannes/attachment-drafts (cf6a2e8) with main (430304f)}

Summary

✅ 50 untouched

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.46%. Comparing base (430304f) to head (cf6a2e8).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5794   +/-   ##
=======================================
  Coverage   88.45%   88.46%           
=======================================
  Files         360      360           
  Lines      100328   100344   +16     
  Branches   100328   100344   +16     
=======================================
+ Hits        88749    88765   +16     
+ Misses       7413     7412    -1     
- Partials     4166     4167    +1     

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[andybalaam]

Looks great, thanks!