Enhance impersonation on Restful API

**Description** I would like one parameter in the REST API configuration object (we can reuse the existing `run_as_user` maybe) to allow me trigger a DAG as an authenticated as long as this "superuser" has a role/policy that allows to impersonate other users. **Use case / motivation** I want to build a task system that submits tasks to Airflow (along with other tools we use, in a SSO way). Users will be authenticated previously when logging to this tasks system and should be able to trigger DAGs. This DAGs have to run AS the user. WEBHDFS (Apache HDFS restful api) achieves something [similar](https://hadoop.apache.org/docs/r1.0.4/webhdfs.html#ProxyUsers). **Are you willing to submit a PR?** I would indeed! It might just take some time since I am fairly new to Airflow **Related Issues** Yes: https://github.com/apache/airflow/issues/982