Support federated or cli credentials for `run-api-scenario` feature

In [this quickstart step](https://github.com/Azure/azure-rest-api-specs/blob/main/documentation/api-scenario/how-to/QuickStart.md#2-create-env-file) we encourage spec authors to create a `.env` file with a `client_id` and `client_secret` defined. This is now disallowed by azure guidelines, so the entire scenario is broken.

We need to update `oav` so that it can recognize either user (eg azcli) or federated auth and leverage the credential in the api scenario codepaths.