Add schema changes for delegate profiling in etdump_flatcc (#280)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/280

Make a few changes to the flatbuffer+python schema to enable delegate profiling

- Remove `EventType` - we can determine the event type by simply checking which of the fields in Event is null.
- Add `instruction_id`, `delegate_debug_id_int` and `delegate_debug_id_str` to `ProfileEvent` to replace `debug_handle`

Reviewed By: Jack-Khuu

Differential Revision: D49076428

fbshipit-source-id: 332c0bf84fb65f04ff6d5d76557d67b08c5f52b3