ES|QL: enable EXPLAIN (snapshot only)

Author: luigidellaquila

x-pack/plugin/esql/qa/server/src/main/resources/explain.csv-spec

@@ -33,12 +33,12 @@ query
     ;
 
 sourceCommand
-    : explainCommand
-    | fromCommand
+    : fromCommand
     | rowCommand
     | showCommand
     // in development
     | {this.isDevVersion()}? timeSeriesCommand
+    | {this.isDevVersion()}? explainCommand
     ;
 
 processingCommand

x-pack/plugin/esql/src/main/antlr/EsqlBaseParser.g4

@@ -1202,7 +1202,12 @@ public enum Cap {
          */
         KNN_FUNCTION(Build.current().isSnapshot()),
 
-        LIKE_WITH_LIST_OF_PATTERNS;
+        LIKE_WITH_LIST_OF_PATTERNS,
+
+        /**
+         * (Re)Added EXPLAIN command
+         */
+        EXPLAIN(Build.current().isSnapshot());
 
         private final boolean enabled;