Add support for delegate profiling in event_tracer (#408)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/408

This diff should most probably be the final event_tracer diff that adds the user-facing API's for delegate profiling. `event_tracer_hooks_delegate.h` can be included by delegate authors in their backend runtime code to get access to these interfaces that will enable them to do profiling logging currently and debugging logging in the future.
- Made changes to `event_tracer.h` to add the hooks to the `EventTracer` class.
- Added `event_tracer_hooks_delegate.h` that contains the public interfaces that delegate authors can call into.

Reviewed By: dbort

Differential Revision: D49360145

fbshipit-source-id: fef2800b2129d93098b8d3868da9007cbc100f4f