Add eligibility group memberships to operator api

[tie-ioki]

• Pays into https://github.com/dbdrive/triebwerk/issues/20300
• Depends on https://github.com/dbdrive/triebwerk/pull/20470 (Do not merge label until this is merged)

[phylor]

This model is a bit convoluted. It combines the schema used in the POST request with the response serializer. We usually only do that if both are named identically and match conceptually (e.g. POST User schema returns a User). In this context, it seems the two models are conceptually different.

Personally I'd split them up:

1. Ioki::Model::Operator::EligibilityGroupMembershipManuallyAssignByEmailAddress
2. Ioki::Model::Operator::EligibilityGroupMembership

You should be able to set the schema as outgoing_model_class in the endpoint configuration.