Move runner-agnostic code out of FlinkJobInvocation

Like BEAM-6714, FlinkJobInvocation contains some code (for asynchronously running jobs) that isn't really Flink specific and therefore could/should be reused by other portable runners. I'm thinking instead of an interface, JobInvocation should be made an abstract class that implements the methods that it currently abstracts, with an abstract method `runPipeline` for the runner-specific details.