Add new duoAssignedUsersCount attribute

Problem

We currently reference Pro in the AiMetics graphql endpoint attributes. This is because we currently only count pro assignment which is being fixed #485506 (closed).

Solution

• Add new duoAssignedUsersCount attribute (backend)

Open questions

• How do we co-ordinate this across BE / FE
• Do we need to ensure backwards compatibility
• Is this a breaking change or do we have the freedom to change it because the API is still marked as an experiment