[iOS] Add the ability to set/unset the focus via JS call

[daniloercoli]

We've added the ability to set/unset the focus via JS calls (ReactNative App -> AztecWrapper) in Android here.

We should add the same feature to iOS, otherwise, if it will ever be used in a RN app without checking the platform will trigger an error.

[koke]

I'm not sure we need to do that, it seems TextInputState is calling UIManager.focus for that on iOS, and I don't think we're calling those methods directly ourselves?