Change delegate debug metadata to use a bytes array + len (#1317)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1317

Currently the debug metadata in event_tracer is expected to be a string. We have come across uses cases e.g. BoltNN profiling where it's useful for this to be an arbitrary sequence of bytes along with the length specified. Updating the API's to reflect this change.

Reviewed By: cccclai

Differential Revision: D51611698

fbshipit-source-id: f4d890c40e06a0e56f6c9129de729aaa52a7f07c