[FEATURE REQUEST] Refactor iOS build and in particular ET setup for iOS build

[mikekgfb]

We should not block improvements to iOS build on a complete refactor to re-use the existing build. That being said this should be a high priority item for future.

Also, I recommend that if for whatever reason this build ends up breaking to not fix it, and do the refactor at that point as the cost is likely equivalent, and the refactor more desirable (plus end to end we should only see half the engineering cost).

As another difference that we might align in the future, my understanding is that we use the latest ET version for iOS, whereas we use a pinned executorch version elsewhere. That may be understandable while the app is still in rapid development and undergoing major improvements (as it has consistently done), but eventually we should follow the same approach for ET build as used for iOS mobile build as for all other uses of ET.