Allow committers only to run GitHub Actions workflows on self-hosted runners

Hi everyone, 

After a meeting held on March 14, 2022 with Gavin McDonald and Jarek Potiuk, we noticed that the implementation of Ash's GitHub Actions Runner [1] would be highly important to have it in the Beam project as well due to security concerns. Ash's version allows us to execute the runners only by approved committers providing us an extra layer of security (this is already implemented in Apache Airflow [2]). 

Currently and with the GitHub Actions Runner [3], everyone can execute runners and workflows with any restriction as it's a public repo. 

We highly recommend incorporating this approach to the current implementation

Thank you!

[1] https://github.com/ashb/runner 
[2] https://github.com/apache/airflow-ci-infra/tree/main/github-runner-ami/packer 
[3] https://github.com/actions/runner