Refactor SamlMembershipRoleSelector component

What does this MR do and why?

Extract a shareable RoleSelector component from the SamlMembershipRoleSelector component so it can be reused for other role selectors in the future.

In addition, use the existing logic for populating member dropdowns.

Issue: #417201 (closed)

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/${group_id}/-/settings/roles_and_permissions and create a custom role with Developer base access
4. Go to http://localhost:3000/groups/${group_id}/-/settings/roles_and_permissions and create a custom role
5. Go to http://localhost:3000/groups/${group_id}/-/saml, fill in the required form fields and select the custom role

MR acceptance checklist

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