In case of you want to setup SAML2 with external IDP using REST callback. You will need to:
- Copy idp-sig.war to $PLATFORM_IDP_HOME/standalone/deployments
- Copy and merge idp-sig-module/module folder with $PLATFORM_IDP_HOME/module

Please read more at: https://github.com/exo-addons/saml2-addon#setup-with-external-idp-using-rest-callback