[executorch][passes] Add config and pass to tag constants for external file

Pull Request resolved: https://github.com/pytorch/executorch/pull/7193

- Add config 'external_constants' to ExecutorchBackendConfig.
- When set to True, run the 'external_constants_pass'
- This tags all constants as external, and moves them into a separate buffer to be serialized outside of the PTE file.

Note: users can write their own passes to tag weights to specific files / multiple files.

TODO: write example pass and test for the case where we have two constant files.
ghstack-source-id: 257353873

Differential Revision: [D66560903](https://our.internmc.facebook.com/intern/diff/D66560903/)

Co-authored-by: lucylq <lfq@meta.com>