Generate OpenAPI definition

[tusmester]

(censored based on discussion)

Create an action ~~(middleware?)~~ that is able to generate an OpenAPI definition file for the repository.

• does not have to contain the full api: basic operations are enough
• API versioning is important
• do we need to generate ~~different definitions based on parameters, or~~ a single file ~~is enough?~~
• ~~OpenAPI definition version 2.0 (swagger, this is what Azure supports) or version 3.0? Or both?~~
• OpenAPI definition version 3.0 (v3 support in Azure).