Add parser to split multi-dimensional 2D spectrum into SpectrumList of 1D spectra

[pllim]

For more details:

• [FEAT] support Spectrum1D objects with 2D flux arrays in Specviz spacetelescope/jdaviz#3223

Think about generalizing and upstreaming from:

• Add ability to load 2D flux arrays in Specviz spacetelescope/jdaviz#3229

[rileythai]

I'm interested in working on this -- do we want this to be a class method or its own separate utility function?

[pllim]

Perhaps @rosteen can advise. Thanks!