Add loader and writer for SpectrumDM

[pllim]

This came up in Astropy Coordination Meeting 2025 breakout session on "stalled development effort." If there is already an issue somewhere, please cross-link and close this as duplicate (I couldn't find it).

Problem: SpectrumDM is a thing, but specutils.Spectrum cannot read files that adhere to this standard.

Work requested:

• Add loader and writer for SpectrumDM.
• Need to improve header manipulation. And writing/validating headers.
• When writing out, emit a warning to the user if a Spectrum object does not adhere to SpectrumDM standard.
• Enable converter. (I do not know exactly what this converter is supposed to do, so hopefully SpectrumDM people can clarify.)

Affected missions:

• Euclid
• SPHEREx

Interested parties: @aragilar @bsipocz @kelle

This issue might also end up in Astropy roadmap. cc @astropy/strategic-planning