TES backend?

I'm exploring the possibility of cwltool being able to submit tasks to a [TES](https://github.com/ga4gh/task-execution-schemas) server as a means of cluster orchestration.

As I understand it, this has been sort of tried before in [funnel-client](https://github.com/bmeg/funnel-client/tree/master/test). There's also possibly something similar in [arvados-cwl](https://github.com/curoverse/arvados/tree/master/sdk/cwl/arvados_cwl). I'm not sure about arvados_cwl, but I've heard it was difficult to keep funnel-client working because the integration with cwltool was tricky.

Mostly I'd like to get the opinion of the devs here who are familiar with the cwltool code. Is there a clean way to write a new executor plugin? Is this something you'd be interested in having in the cwltool codebase?