Assign stable IDs to ConsumerGroups

[czeslavo]

Is there an existing issue for this?

• I have searched the existing issues

Problem Statement

After introducing ConsumerGroups described in the RFC, we should use the very similar mechanism for assigning their IDs explicitly when FillIDs feature gate is turned on that was implemented in #3906 for Services, Routes, and Consumers.

Proposed Solution

• Add ConsumerGroup.FillID method to go-kong (similarly to feat: add FillID method for Service, Route and Consumer go-kong#299).
• Use the method in KIC when FillIDs feature gate is turned on

Additional information

No response

Acceptance Criteria

• As a user, when I create a ConsumerGroup using Kubernetes CRD and I have FillIDs feature gate turned on, I want the ConsumerGroup to always be assigned a stable ID based on its name so that it's the same across Gateway instances that KIC configures.

[czeslavo]

Blocked by #3728.