Support add_ssh_key on self-hosted runners
To reach parity with existing non-runner jobs, the
add_ssh_key(https://circleci.com/docs/2.0/configuration-reference/#add_ssh_keys) step should be supported.
This will allow use of additional configured keys for pushing tags and generated-commits.
The key should be added in a way that remains scoped to the runner job - and not globally on the hosting machine.