Add group SAML configuration changes to group audit events

We should track when changes are made to a Group SAML configuration in the group audit log. This includes:

• Assertion service URL
• Identifier URL
• Enable/disable toggle
• SSO URL
• Cert
• SSO-SAML Group Sync configurations at group and subgroups level

Implementation plan

• Add callbacks to SamlProvider services/controller/model to track changes to the above attributes.

Useful links

• https://docs.gitlab.com/ee/development/audit_event_guide/#audit-event-guide