Add an audit event for user provisioning by group SCIM

What does this MR do and why?

The MR adds an audit event type user_provisioned_by_scim that is triggered when a user is provisioned by SCIM.

References

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How to set up and validate locally

• On a self-managed instance, configure group SAML along with SCIM
• Patch your instance to include the changes to files ee/lib/ee/gitlab/scim/base_provisioning_service.rb and ee/config/audit_events/types/user_provisioned_by_scim.yml
• Pick a user that has SAML identity but not SCIM identity.
  • (you can remove a SCIM identity for a user via rails console by running: ScimIdentity.find_by_user_id(<user_id>).destroy! or ScimIdentity.find_by(extern_uid: <SCIM Id>).destroy!
• On the IdP, trigger a provision for the user and notice the audit logs for the group. If provisioning is successful:
  • You will find a record for that user with action User was provisioned by SCIM
  • You will find the user now has a SCIM identity tied to their account.