Add optional parameters to `fetch`

[rplzzz]

The idea here is that a capability could be defined with zero or more optional parameters. These parameters could do things like subset a dataset so that you don't have to retrieve the whole thing (possibly useful when dealing with huge outputs).

It would require a little fancy footwork to implement. Notably, we would have to add a method that applies optional arguments to the data to the interface, but it might be worth it if it allows us to manage the amount of data that we have to absorb in a single go.