Check the http request header to match on a regex string.
Initial code by Sebastian Preisner, copied from keycloak/keycloak#14605.
Relevant Keycloak Documentation part: https://www.keycloak.org/docs/latest/server_admin/#conditions-in-conditional-flows
Use with Maven (>3) and JDK (>= 17).
mvn clean verify
Copy deployments/conditional-http-header-authenticator-jar-with-dependencies.jar to /providers/ directory in Keycloak.
See https://github.com/elexis/keycloak-auth-deny-user-nosecondfactor for a usage example.