Add member_role_id to saml_providers

What does this MR do and why?

This adds the member_role_id column to the saml_providers table.

When a member role is defined, it allows for creating members with custom roles through Group SAML and/or SCIM (re-)provisioning.

How to set up and validate locally

1. Create a group with Ultimate license
2. Configure SAML for the group
3. Create a custom role for the group (Make sure the base role matches the Group SAML's Default membership role).
4. In Rails console, set the new created member_role of the new created saml_provider:

   SamlProvider.last.update(member_role: MemberRole.last)

5. Create a new user by signing in using the SAML SSO link and verify the member_role_id is set on the user's member record:

   User.last.members.last.member_role

Issue: #417285 (closed) (Backend part)

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