Make SubmodelDescriptor/id optional

[BirgitBoss]

What?

There are two valid ways to implement AAS registries:

• AAS Registry including SubmodelDescriptors
• AAS Registry containing AAS endpoint but no SubmodelDescriptors + separate Submodel Registry

In the first case the ID of the Submodel is not needed because it is implicitly contained in the endpoint. Many operators probably would also prefer to hide this (internal) Submodel/id.

How?

Make SubmodelDescriptors/id optional

• add constraint for SubmodelRegistry that it is mandatory for it

Or

Make two SubmodelDescriptors classes, one for AAS registry and one for Submodel Registry

This change is backward compatible

• I have signed the required Developer Certificate of Origin (DCO) already.