Implement a way to configure ssh for self-hosted runner

Currently, when using self-hosted runners, the SSH directory is mounted to the build container inside a tmp folder in the directory /tmp/<runner_uuid>/ssh.

Since the runners uuid changes from one runner to another, the path to the SSH folder also changes from runner to runner, relying on that path to configure SSH (e.g fetching fingerprint from a private IP address in the runner network) can be fragile as it would rely on regex and also we can’t guarantee that path won’t be changed in the future.

This feature request is to provide a way of configuring SSH for a runner. Some alternatives are :

• mount ssh folder in a build container to /opt/atlassian/pipelines/agent/ssh 
•  provide a way to configure runners and provide their own ssh config
• change the existing SSH configuration in pipelines and allow to provide fingerprint manually in the UI.