[ios] guideline on iOS engine feature flags: Info.plist vs cmd flag

### Use case

There are currently 2 ways of setting the flag on iOS: via cmd line flags or via Info.plist. It's unclear to me which one to use, as I see pros and cons on both.

Also there's an inconsistent practice: on android, `systrace` is enabled via cmd flag([see](https://github.com/flutter/flutter/blob/d70c9528f8a18b7ca7fdf5fde723e4ad7fcf23c3/engine/src/flutter/shell/common/switches.h)), but on iOS it's enabled via Info.plist ([see](https://github.com/flutter/flutter/blob/d70c9528f8a18b7ca7fdf5fde723e4ad7fcf23c3/engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterDartProject.mm#L180))

### Proposal

Come up with a guideline for the future.