[Core] SkyPilot should support arbitrary SSH targets

[colinjc]

In some cases we may have access to GPUs that aren't a part of existing supported clouds. For example, we may be using a new provider, or even just have access to a local machine. If SkyPilot could be configured to connect to arbitrary SSH targets, we would be able to access these GPUs, and burst out to other clouds if they're occupied.

[Michaelvll]

Related to #3926