Define the min/required API a Spectrum meta data object

[samaloney]

Describe the feature

Need standardised attribute/method to code against a good example if the Sun-Observer distance and the angle for the Albedo model. If the meta data had .observer_coordinate and target_coordinate these can be derived on the fly.

Related to #258

Similar to:

• https://github.com/sunpy/sunraster/blob/main/sunraster/meta.py
• Add spectrogram metadata classes radiospectra#245

Proposed solution

No response