Adding KQL_FUNCTION capability.

Author: afoucret

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/action/EsqlCapabilities.java

@@ -404,6 +404,11 @@ public enum Cap {
          */
         MATCH_FUNCTION,
 
+        /**
+         * KQL function
+         */
+        KQL_FUNCTION(Build.current().isSnapshot()),
+
         /**
          * Don't optimize CASE IS NOT NULL function by not requiring the fields to be not null as well.
          * https://github.com/elastic/elasticsearch/issues/112704