[ephemeral] Add option ephemeral runners

[npalm]

As part of #1372 we first the option to enable ephemeral runners.

The option will be only enable_ephemeral_runners will be by default set to false. Once switch to true in combination with the workflow_job event runners will be created with --ephemeral once the job is finished the instance will be terminated.

[keenan-v1]

So glad to see this is coming. I'm researching self-hosted runners, so I'm curious what the ETA might be for this feature?

[npalm]

No ETA yet on best effort, but the goal is to implement the different parts separatly. First making ephemeral possible as a kind of beta feature. Next also find a way ot have a pool to start builds faster, and a base ami. Any help is welcome

[ScottGuymer]

Working on adding a pre-built AMI in #1444