Group members Vue conversion - role dropdown

What does this MR do?

Part of &4233 (closed) and #235603 (closed)

I am working on converting the group members page from HAML to Vue. This MR adds the role dropdown to the members table. Changing the role does not currently do anything. To keep this MR small the functionality that makes the API request when role is changed will come in a subsequent MR.

Currently this is behind the vue_group_members_list feature flag. To enable:

1. bin/rails console
2. Feature.enable(:vue_group_members_list)

Review app

https://gitlab-review-235603-con-4lz701.gitlab-review.app/groups/gitlab-org/gitlab/-/group_members

Local testing

1. Create a group
2. Navigate to "Group" -> "Members"
3. Add a user to the group using the "Search for members to update or invite" input
4. Role dropdown should appear next to newly added user

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
  • Not needed, behind a feature flag
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team