Call generate_api tool in pullrequest pipeline to make sure the API.md file is up to date

We need to call `generate_api` in pull request pipelines to make sure the `api/API.md` file was updated. This should be called in the `rust - pullrequest` pipeline under `eng/pipelines` and needs to work in network isolation. If necessary, add a `--check` argument like some other tools to make sure the file is saved to a temp location and then both that and the final `api/API.md` file are hashed and hashes compared for equivalency. If the hashes differ, write an actional error message so that GitHub displays it that `generate_api` needs to be called with the correct arguments (the error message should include the exact command line string including the crate name(s)) to update `API.md`.