Add api for dictionary with kernel metadata (#1334)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1334

- Add new API for selective build to specify ops and metadata
- Metadata follows [this format](https://www.internalfb.com/code/fbsource/[117d4d1edb0f]/fbcode/executorch/runtime/kernel/operator_registry.h?lines=97)

Can potentially update usages of the list API to this.

Reviewed By: larryliu0820

Differential Revision: D51698159

fbshipit-source-id: d27a781f7c86b53e3c4dc557a9131ad6e5860deb