Surface delegate_debug_metadata to Event (#1131)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1131

The delegate_debug_metadata field exist in [etdump schema](https://fburl.com/code/n6sw6es8), but hasn't been surfaced to the Inspector

Reviewed By: tarun292

Differential Revision: D50817817

fbshipit-source-id: f784cb81a58fce0cc089e9d7ae550dfaa5bf2fac