OpenApi: Rework interaction with OpenApiObject

We interact with OpenApiObject in unfortunate way:

1. we are creating OpenApiObject.Builder several times
2. then we are mutating schemas
3. then we are mutating paths
4. we are relying on side effects (fillDoc method returning void)

We can make better if we try to rework our logic to prepare schemas ans paths beforehand and then just create one OpenApiObject instance.