Update function `extract_bounding_spectral_region` so as to handle spatial regions as well

Function `extract_bounding_spectral_region` is supposed to replace `minimal_subcube` in `spectral-cube`, but falls short of that. It handles only the spectral dimension. It needs to be extended to handle spatial regions as well.

Note that this only makes sense when masks are supported for spectral cubes. In the spectral domain `SpectralRegion` acts as as _de facto_ mask, but in the spatial dimensions the only way to generate an "empty" region is via masking.

Locate the `extract_bounding_spectral_region` entry point and compare its current behavior with `minimal_subcube`. Read how `SpectralRegion` and masks are handled for spectral cubes, then determine the spatial-region cases that need coverage. Done means the function handles supported spatial masks as well as spectral regions without changing its existing spectral behavior.

Written by the indexing model from the issue text.