[FE] Add custom roles to policy drawer

Why are we doing this work

• users want to see their custom roles in the policy drawer

Relevant links

Non-functional requirements

• Documentation:
• Feature flag:
• Performance:
• Testing:

Implementation plan

• update policy_approvals.vue to request custom roles and map them to the ids in the policy

Verification steps

1. Upload a GitLab Ultimate license
2. Navigate to a project/group => Secure => Policies => New policy => Merge request approval policy
3. Create a new policy with some custom roles
4. Navigate the project/group => Secure => Policies => Select the new policy
5. Verify the policy drawer shows the name of the custom role