[executorch][flat_tensor] flat tensor header (#7869)

Pull Request resolved: https://github.com/pytorch/executorch/pull/7764

Parse a FlatTensor header. The FlatTensor header contains:
- flatbuffer offset
- flatbuffer size
- segment base offset
- segment data size (total size of all segments)

Mostly taken from extended_header.cpp/.h
ghstack-source-id: 262588577

Differential Revision: [D67064570](https://our.internmc.facebook.com/intern/diff/D67064570/)

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