Add support for event_tracer in KernelContext (#344)

Summary:
X-link: https://github.com/pytorch/pytorch/pull/109337

Pull Request resolved: https://github.com/pytorch/executorch/pull/344

This diff does a couple of things:
- Adds a pointer to an instance of EventTracer inside `KernelRuntimeContext`
- Adds an instance of `EventTracerProfileScope` inside the codegen layer.

Currently calls to both `EventTracerProfileScope` and `EXECUTORCH_SCOPE_PROF` are present inside the codegen layer. This is a temporary state. Once EventTracer and ETDump has been fully integrated, the call to `EXECUTORCH_SCOPE_PROF` will be removed everywhere. `EventTracerProfileScope` should be a no-op for now.

Reviewed By: dbort

Differential Revision: D48975975

fbshipit-source-id: edd4d674e25453e6f086399dc9eba818441c04f3