Add support for delayed event (MSC4140)

[BillCarsonFr]

Motivation: I am working on a rust rtc stack https://github.com/BillCarsonFr/matrix-rust-rtc. , and some of the needed primitive are missing in rust-sdk.

I am starting with support of delayed event (state events and room event).
Currently the widget driver had some support for it (by using raw ruma requests), and I am moving it so it became part of the API.

While doing so I noticed that the driver implementation was faulty:

• It was not encrypting delayed events in encrypted rooms 🥶
• It was not checking for delayed event support before processing. This is annoying because delaying an event is just adding a query param to the send request, so if the homeserver ignores it there is a risk of sending the delayed event immediatly. Hopefully recent synapse detects that and errors with M_FORBIDDEN: MatrixError: [403] Sending delayed events has been disallowed

A note on the send event future, I cannot just add an optional parameter to SendRawMessageLikeEvent because delayed event returns a delayed_id, and the send event future returns a OwnedEventId
In order to share the encryption code, I created an intermediate future PreparedMessageLikeEvent that handles the common part before the ruma request.
I added a SendRawMessageLikeEvent::with_delay that converts to the new SendDelayedRawMessageLikeEvent future

• I've documented the public API changes in the appropriate changelog files (see Writing changelog entries).
• This PR was made with the help of AI.

Signed-off-by:

[codecov]

Codecov Report

❌ Patch coverage is 86.44068% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.23%. Comparing base (7def4e6) to head (cfb09a4).
⚠️ Report is 11 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/room/futures.rs	81.42%	9 Missing and 4 partials ⚠️
crates/matrix-sdk/src/client/mod.rs	75.00%	0 Missing and 2 partials ⚠️
crates/matrix-sdk/src/widget/matrix.rs	91.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6973   +/-   ##
=======================================
  Coverage   90.23%   90.23%           
=======================================
  Files         408      408           
  Lines      117281   117352   +71     
  Branches   117281   117352   +71     
=======================================
+ Hits       105823   105893   +70     
- Misses       7511     7521   +10     
+ Partials     3947     3938    -9     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing valere/delayed-event-api (cfb09a4) with main (f4b9512)}