container/job executor abstraction

Like with real gitlab, it is possible to run the whole job on another machine/container.

Currently Job and DockerJob contain implementations of the actual execution (and container startup) and interactive stubs.

Break this out into another class that presents an abstract interface the jobs can call into.

This could be used to execute jobs on kubernetes, podman, docker via ssh, remote docker (eg remote windows docker) etc

Idea: This could be done by emulating the runner job request API.