Allow backends to implement either create_instance() or run_job() #887

With the introduction of pools in #880, backends may opt-in to implement create_instance method to support dstack pool add. Such backends are still required to implement run_job() although this requirement is unnecessary. The proposal is to let backends implement either create_instance() or run_job() depending on the backend type. To make this more straightforward for contributors, we may introduce VMCompute and ContainerCompute base classes that would require create_instance() and run_job() respectively.

Another related idea to improve Compute interface is to separate optional methods like create_gateway() in mixins like ComputeWithGateway.