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

extension/flat_tensor/test/flat_tensor_header_test.cpp

@@ -34,6 +34,7 @@ class FlatTensorHeaderTest : public ::testing::Test {
  * this test data must change too. The layout of the header is a contract, not
  * an implementation detail.
  */
+ // clang-format off
 // @lint-ignore CLANGTIDY facebook-hte-CArray
 constexpr char kExampleHeaderData[] = {
   // Magic bytes