Handle XNNHeader in XNNPACK Runtime (#1543)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1543

We introduce XNNHeader on runtime side to handle the newly introduced XNNHeader ahead of time. XNNHeader manages the offsets and sizes of the flatbuffer payload and the constant data payload so that it is accessible by the XNNCompiler

It is important to note that on serialization side, we have not yet switched our serialization method to `serialize_xnnpack_binary` so this does not yet use the new serialization format. However, passing tests on this illustrates BC as old models will still be able to run on this new runtime.

Passing tests here show that the Header Magic correctly works in discerning between using the XNNHeader and the Flatbuffer header

Reviewed By: digantdesai

Differential Revision: D52556131

fbshipit-source-id: 240e88f891d9b4ec48ab7a8b6c93aaddf0bacdde