Allow for interpolation in `rectify_dataset()`

[forman]

Is your feature request related to a problem? Please describe.

Do to a recent use case new requirements emerged for spatial resampling :

1. Enhance function xcube.core.resampling.rectify_dataset() by a linear interpolation between adjacent source pixels.
2. Allow passing keyword arguments for xcube.core.resampling.rectify_dataset() to xcube.core.resampling.resample_in_space().

Describe the solution you'd like

1. Introduce optional keyword argument interpolation with values "nearest" and "linear" and implement the linear interpolation in _compute_var_image_xarray_dask() and _compute_var_image_xarray_numpy()
2. Introduce optional keyword argument rectify_kwargs in xcube.core.resampling.resample_in_space()
3. Deprecate keyword argument xy_var_names unused for a long time from function xcube.core.resampling.rectify_dataset().