[Frontend] Add filtered list of groups to a dropdown

Why are we doing this work

1. Policy scope will now have fourth option: scoping to a group
2. List of loaded groups should be limited to linked to same SPP as opened policy, according to the discussion

Relevant links

1. Discussion

Non-functional requirements

• Documentation:
• Feature flag: policy_group_scope
• Performance:
• Testing: unit tests

Implementation plan

1. Update ee/app/assets/javascripts/security_orchestration/components/group_projects_dropdown.vue to load only restricted lists of ids
2. Update ee/app/assets/javascripts/security_orchestration/components/policy_editor/scope/scope_section.vue to load linked namespaces for groups as well, when all linked groups option is selected, path groups to group_projects_dropdown.vue

Verification steps