Create new REST API endpoints to support self-managed

1. Create new instance-level endpoints to match the functionality of our existing (group level) member role endpoints.

• List all member roles of the instance
• Add a member role to the instance
• Remove member role of the instance

2. Ensure the assignment REST endpoints work with instance level member roles:

• https://docs.gitlab.com/ee/api/members.html#add-a-member-to-a-group-or-project
• https://docs.gitlab.com/ee/api/members.html#edit-a-member-of-a-group-or-project

3. Ensure existing group level endpoints will fail "the right way" for self-managed

What happens when the existing group-level endpoints are hit? It's possible we'll want to provide a specific error message for self-managed users who are using the group level endpoints and recommend the new endpoint.

API docs: https://docs.gitlab.com/ee/api/member_roles.html

Related Discussions: #429280 (comment 1801125237) #429280 (comment 1800280369)