Add Labs screen for beta testing of public features (#5465)

* Add Labs screen:

- Make `Feature` have an `isInLabs` boolean to distinguish private feature flags from public ones.
- Have `FeatureFlagsService` provide the list of available flags.
- Display the labs item in the settings screen only if there are available public features.
- Remove public feature toggles from developer options.
- Implement the labs screen with the public features.
- Add a clear cache step to the threads feature toggle
- Update screenshots

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Co-authored-by: ElementBot <android@element.io>

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