Passing keyword arguments to external functions

What is your issue?

Follow on from #6891 and #6978 to discuss how we could homogenize the passing of keyword arguments to wrapped external functions across xarray methods.

There are quite a few methods like this where we are ultimately passing data to numpy, scipy, or some other library and want the option to send variable length kwargs to that underlying function. There are two different ways of doing this today:

1. xarray method accepts flexible **kwargs so these can be written directly in the xarray call
2. xarray method accepts a single dict kwargs (sometimes named differently) and passes these along in expanded form via **kwargs

I could only find a few examples of the latter:

• Dataset.interp, which takes kwargs
• Dataset.curvefit, which takes kwargs (although the docstring is wrong here)
• xr.apply_ufunc, which takes kwargs passed to func and dask_gufunc_kwargs passed to dask.array.apply_gufunc
• xr.open_dataset, which takes either **kwargs or backend_kwargs and merges the two

Allowing direct passage with **kwargs seems nice from a user perspective. But, this could occasionally be problematic, for example in the Dataset.interp case where this method also accepts the kwarg form of coords with **coords_kwargs. There are many methods like this that use **indexers_kwargs or **chunks_kwargs with either_dict_or_kwargs but don't happen to wrap external functions.

Start by comparing the keyword-argument patterns described for Dataset.interp, Dataset.curvefit, xr.apply_ufunc, and xr.open_dataset, including their interactions with coordinate and backend keyword arguments. Done requires an agreed, consistent API approach across wrapped external functions, with the affected methods and documentation identified.