Serialize constant Data outside of flatbuffer (#1542)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1542

We introduce the `serialize_xnnpack_binary` method which serializees the constant data outside of the flatbuffer. It leverages the xnnheader introduced in the previous diff to store offsets and sizes for both the flatbuffer payload as well as the constant data payload.

Note here we have not yet switched the delegate to use the new `serialize_xnnpack_binary` function as this new serialization also requires changes on the runtime side. This will be tested in the diff which follows.

Reviewed By: digantdesai

Differential Revision: D52498367

fbshipit-source-id: d90a246630653167ec479913f1862851ab8ace82