feat: add sticky events to SyncUpdate

[giles-labrys]

Add support for Sticky Events to the SyncUpdate class used to model the response of the /sync endpoint.

Specifically this is so that its possible to follow the MSC4143: MatrixRTC specification on how to initiate and manage MatrixRTC sessions using the "slot" state event and new concept "sticky" events. The slot state event is not part of this PR to keep it super small and focussed, though that change is also small and can be followed up in another PR (just adding a new EventType for it following the msc_extension directory pattern).

I've not attempted to do an msc_extension approach for sticky events as they are retrieved through the /sync endpoint and it doesn't seem feasible to implement it any other way. These events are nullable and so there is no risk of regression and the MatrixRTC proposal itself is quite stable (though not officially merged yet).

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

❌ Patch coverage is 3.22581% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.80%. Comparing base (574500e) to head (6d4f0b4).
⚠️ Report is 63 commits behind head on main.

Files with missing lines	Patch %	Lines
.../msc_extensions/msc_4354_sticky_events/models.dart	0.00%	28 Missing ⚠️
lib/matrix_api_lite/model/sync_update.dart	33.33%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2238   +/-   ##
=======================================
  Coverage   57.80%   57.80%           
=======================================
  Files         158      159    +1     
  Lines       19679    19756   +77     
=======================================
+ Hits        11375    11420   +45     
- Misses       8304     8336   +32     

Files with missing lines	Coverage Δ
lib/matrix_api_lite/model/sync_update.dart	44.38% <33.33%> (-0.19%)	⬇️
.../msc_extensions/msc_4354_sticky_events/models.dart	0.00% <0.00%> (ø)

... and 6 files with indirect coverage changes

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[td-famedly]

can you please put the model in the msc_extensions folder? I think it's a bit cleaner this way, as this is still a msc. Having sticky as a paramter in /sync is fine then

[giles-labrys]

Done thank you 🙏 - although I think I raised this PR prematurely anyway as I don't think the latest matrix server version I'm working with supports sticky events yet. I can mark this PR as draft or close it and then re-open later when the functionality of the msc is usable?

Edit: Release 1.150 included "Expose matrix-org/matrix-spec-proposals#4354 over the legacy (v3) /sync API. (element-hq/synapse#19487)" so this can now be used.

[giles-labrys]

Hi @td-famedly please can you re-review this?

Release 1.150 included "Expose matrix-org/matrix-spec-proposals#4354 over the legacy (v3) /sync API. (element-hq/synapse#19487)" so the changes here can now be taken advantage of