[SpawnProcess] Add ProcessSpec to StreamingLogHandler

[aciidgh]

This will allow clients to distingush streams coming from different spawn processes.

[rballard2]

The intended way to distinguish streams from different SpawnProcesses is to pass different stdoutStreamingDestination/stderrStreamingDestination for different invocations. Is there somewhere where this wouldn't work?

[aciidgh]

that would also work but it seems less ergonomic to have to specify unique filenames for all clients

[rballard2]

Fair point. How about autogenerating a unique name by default? Something like "\(executable)/\(hash(arguments)).(log|err)" for the output/error logs? That's hopefully a good mix of human-readable and reasonably-unique, and you can still override the log names if you need to.

[aciidgh]

That's a good idea but I think we'll still want the spec object because clients will want to know what process is being executed and what were the args.