create belgif-openapi-bom

[jpraet]

A Maven BOM could be created to keep the versions of the belgif-openapi types in synch.

It can be created here: https://github.com/belgif/openapi-bom

Some additional thought is required about the versioning of this BOM:

• we could increase the minor version each time the minor version of a module is increased, and the major version each time the major version of a module is increased
• or, a date could be used as version number, e.g. belgif-openapi-bom 20210712

[pvdbosch]

I see some potential maintenance problems if we create a BOM: if the use case is to easily let projects update to the most recent patch versions, and we have N belgif-openapi modules with each M major versions, we'd need to maintain MxN branches of the BOM.

If we don't maintain these branches in order to provide patches, we'd make updating more difficult. And unlike BOMs for libraries like spring cloud or JEE, it's not that easy to drop support for older schema types.