Improve offerings

[jelmerderonde]

This PR improves the way the specification is written with regards to Offerings. It makes several changes:

1. Update the OpenAPI version to 3.1.0 to allow the use of the const type.
2. Make offeringType a constant set to program, course or component in the respective components.
3. Restructure various files to simplify file organization.
4. Reoorder attributes so the documentation renders the attributes in a more logical way.

[hamrt]

Looks good to me I'm not sure how breaking this is though and if it should be part of a 5.0 patch round or more for an 5.1 or even 6.0

[jelmerderonde]

@hamrt I don't think this is breaking or even changing the (intended) spec in any way. This PR is intended to merely write down the spec as we know it in a more precise way. Maybe we should meet to discuss?