Wrap server generated yaml files inside et_operator_library (#5778)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5778

## This diff
This diff is to make the model config codegen flow work on Edge MLHub by enabling the `et_operator_library` API to take a .yaml file.

- The code changes in `fbcode/executorch/codegen/codegen.bzl` was copied from a stale diff D61824614 made by Mengwei. That one didn't landed because the owner of SceneX/v5002 model didn't have time to test it.

## Context
By design, this is the codegen flow:
1. Use click the "Generate ET model config" button on Edge ML Hub;
2. Two files will be generated, one is a .yaml with a list of kernels, the other is a BUCK file under path `xplat/executorch_model/build/...` that calls `et_operator_library` and pass in the .yaml as a parameter. See D63667456 for an example;
3. A bot would accept and land the diff if CI passes;
4. User can run inference.

Reviewed By: larryliu0820

Differential Revision: D63305843

fbshipit-source-id: 18ceea47b4de81f1a52eb58ce43a4307b5f3cfee