[Story] Show advanced settings for external users

[VolkerJunginger]

Story

As an external user that wants to configure their element client beyond the default setting I would love to access certain feature flags in a separated settings view.

• Create an "Advanced settings view" for external users. In the settings view.

The following options should be configurable:

• Message Layout (Bubbles vs. Modern)
  -- Use the existing setting and move it here
• Enable Rich Text Editor
• • Use the existing setting from developer options and move it here @langleyd Good idea or should this be an other one?
• View event source option
• • Needs to be build. Enables to see the event source option on long pressing an event.

Scope

### Android
- [ ] https://github.com/vector-im/element-x-android/issues/1317

iOS

- [ ] https://github.com/vector-im/element-x-ios/issues/1699

• QA signoff on completion
• Design signoff on completion
• Product signoff on completion

Stretch goals

None at this time

Out of scope