Add helpers to list implementations+specs

[LukeWeidenwalker]

Address issue raised in Open-EO/openeo-python-client#411 (comment).

Unless I'm missing something, I think this can be done more compactly with:

> for name, func in inspect.getmembers(
>     openeo_processes_dask.process_implementations,
>     inspect.isfunction,
> ):
>     process_registry[name] = Process(
>         spec=getattr(openeo_processes.specs, name),
>         implementation=func
>     )
> Also, wouldn't it be better that openeo_processes_dask provide helpers to list implementations/spec, instead of having to use ugly inspect.getmembers and getattr constructs?

[LukeWeidenwalker]

Another comment here is that currently the submodule specs includes specs that aren't actually implemented here.