Add actors to add on assignment feature flags

What does this MR do and why?

Addresses: https://gitlab.com/gitlab-org/gitlab/-/issues/427034+

This MR:

1. Adds the namespace actor to the existing hamilton_seat_management feature flag
2. Adds a new feature flag code_suggestions_user_assignments to transition the user's access to be governed by assignment records
   • Rollout issue: https://gitlab.com/gitlab-org/gitlab/-/issues/428574

We want to be able to roll this out to specific actors for verification, and then also using a percentage rollout on production. Add the actor to the feature flags to give us this ability.

How to set up and validate locally

Start the GDK, CustomersDot and ZSim

Existing Behaviour

1. Create a new group
2. Follow the instructions to create an add on purchase for that group
3. Add a developer to the group
4. Find that developer in the console and verify they have code suggestions access:

   developer_user.code_suggestions_add_on_available?
   => true

New Behaviour

5. Turn on the code_suggestions_user_assignments FF
6. Find the developer in the console and verify they don't have code suggestions access:

   developer_user.code_suggestions_add_on_available?
   => false

7. Turn on the hamilton_seat_management FF
8. Navigate to the usage quotas page for the group
9. Assign the developer user the add on
10. Find the developer in the console and verify they do have code suggestions access:

    developer_user.code_suggestions_add_on_available?
    => false

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.