refactor(send_queue): vectorize media handles on SendHandle

[Johennes]

This was broken out of #4838 and is a preliminary step towards implementing MSC4274. The media_handles field on SendHandle is turned into a vector so that it can hold handles for several media when upload a gallery later.

• Public API changes documented in changelogs (optional)

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.71%. Comparing base (5753ca3) to head (64bf57e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/send_queue/mod.rs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4898   +/-   ##
=======================================
  Coverage   85.70%   85.71%           
=======================================
  Files         309      309           
  Lines       35387    35389    +2     
=======================================
+ Hits        30330    30332    +2     
  Misses       5057     5057           

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[Hywan]

Makes sense to me. Approving. Thanks for your patch!