Add a feature flag for Simplified Sliding Sync.

[pixlwave]

Requires #3056 and matrix-org/matrix-rust-sdk#3723 (plus an SDK release).

[github-actions]

	Warnings
⚠️	Some of the commits are missing ticket numbers. Please consider squashing all commits that don't have a tracking number.
⚠️	You seem to have made changes to views. Please consider adding screenshots.

Generated by 🚫 Danger Swift against 44a050e

[stefanceriu]

lgtm 👍

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 21 lines in your changes missing coverage. Please review.

Project coverage is 77.88%. Comparing base (288d2c2) to head (44a050e).

Files	Patch %	Lines
...perOptionsScreen/View/DeveloperOptionsScreen.swift	0.00%	7 Missing ⚠️
NSE/Sources/NotificationServiceExtension.swift	0.00%	4 Missing ⚠️
...ources/Services/UserSession/UserSessionStore.swift	0.00%	3 Missing ⚠️
NSE/Sources/Other/NSEUserSession.swift	0.00%	2 Missing ⚠️
ElementX/Sources/Application/AppCoordinator.swift	0.00%	1 Missing ⚠️
ElementX/Sources/Application/AppSettings.swift	0.00%	1 Missing ⚠️
...mentX/Sources/Other/Extensions/ClientBuilder.swift	0.00%	1 Missing ⚠️
...ervices/Authentication/AuthenticationService.swift	0.00%	1 Missing ⚠️
...X/Sources/Services/QRCode/QRCodeLoginService.swift	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3057      +/-   ##
===========================================
- Coverage    77.96%   77.88%   -0.09%     
===========================================
  Files          694      694              
  Lines        54046    54099      +53     
===========================================
- Hits         42138    42136       -2     
- Misses       11908    11963      +55     

Flag	Coverage Δ
unittests	68.61% <0.00%> (-0.04%)	⬇️

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud