Split `Resource.h` into multiple files within `memory/` (#3553)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3553

This change is a no-op and simply refactors existing classes.

I'm trying to learn what's going on within this `api/` folder.

`Resource.*` files can be split to be less intimidating to readers. I'm also thinking we can flatten the hierarchy more in future changes.
ghstack-source-id: 225755561
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Reviewed By: SS-JIA

Differential Revision: D57126893

fbshipit-source-id: 0a5f729c70351a62647af70225d8288faa5af035