Add saving a custom role to SAML Group Links

What does this MR do and why?

This Vue-ifies the SAML Group Links Role selector and adds support for saving and displaying a custom role.

Issue: #417201 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable Group SAML

   gdk config set omniauth.group_saml.enabled true && gdk reconfigure && gdk restart rails-web

2. Enable the feature flag in the console:

   Feature.enable(:custom_roles_for_saml_group_links)

3. Create a group with Ultimate license

4. Go to http://localhost:3000/groups/${new_group}/-/settings/roles_and_permissions and create a custom role

5. Go to http://localhost:3000/groups/${new_group}/-/saml and enable SAML authentication by adding a bogus identity provider:

   

6. Go to http://localhost:3000/groups/${new_group}/-/saml_group_links, fill in a name and select the custom role

7. Verify the SAML Group Link was saved successfully with the custom role

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.