Annotation for defining Pydantic types based on AstroPy's Quantity

[hettlage]

This PR addresses issue #55.

It adds a new class AstropyQuantityTypeAnnotation, which lets the user define custom types making use of AstroPy's Quantity class.

Tests for this new class are added, and the documentation on defining models is updated.

[Fingel]

Hello again @hettlage ! Thanks for this PR.
It does look like a nice type to have. It doesn't appear to be used anywhere. Is this intentional? For example, in the proper motion field for Targets:

AEONlib/src/aeonlib/models.py

Line 33 in 6eb80fe

proper_motion_ra: Annotated[float, Le(20000.0)] = 0

[hettlage]

I'm happy to update the models. This will need some additional validators, as the validators of the annotated_types library don't support Quantity objects. I'll extend the PR accordingly.

[hettlage]

I have added some validators.

But on second thoughts, I'm reluctant to update the existing models, as this would be a breaking change - consumers of the models might assume that the affected properties have float values, whereas now in fact they would be Quantity objects.

Would you be happy with such breaking changes?

[hettlage]

Is there anything else you would need from me for this PR? :-)