Create separate ETDump schema for FlatCC (#214)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/214

Create separate ETDump schema for FlatCC to enable us to modify the schema without impacting the default FlatC implementation. This allows us to modify the schema with FlatCC-specific considerations.

Reviewed By: tarun292

Differential Revision: D48743188

fbshipit-source-id: 04d94f29d895fa58b298ccaf87bf03a88552bda9