Vueify default_membership_role_dropdown and add custom roles

What does this MR do and why?

This vueifies the default_membership_role_dropdown selector and allows for custom roles to be selected and saved to SAML settings.

Issue: #417285 (closed) (Frontend part)

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. Create a group with Ultimate license
3. Go to http://localhost:3000/groups/${new_group}/-/settings/roles_and_permissions and create a custom role
4. Go to http://localhost:3000/groups/${new_group}/-/saml, fill in the required form fields and select the custom role
5. Reload the page and verify the custom role has been saved

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.