Update base for Update on "[executorch][flat_tensor] flat tensor header"

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

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

[ghstack-poisoned]

Author: lucylq