Add explain request handlers.

Signed-off-by: Yury-Fridlyand <yury.fridlyand@improving.com>

core/src/main/java/org/opensearch/sql/executor/execution/PaginatedPlan.java

@@ -39,6 +39,7 @@ public void execute() {
 
   @Override
   public void explain(ResponseListener<ExecutionEngine.ExplainResponse> listener) {
-    throw new UnsupportedOperationException("implement PaginatedPlan.explain");
+    listener.onFailure(new UnsupportedOperationException(
+        "`explain` feature for paginated requests is not implemented yet."));
   }
 }

sql/src/main/java/org/opensearch/sql/sql/SQLService.java

@@ -67,6 +67,12 @@ private AbstractPlan plan(
       Optional<ResponseListener<ExplainResponse>> explainListener) {
     if (request.getCursor().isPresent()) {
       // Handle v2 cursor here -- legacy cursor was handled earlier.
+      if (queryListener.isEmpty() && explainListener.isPresent()) { // explain request
+        explainListener.get().onFailure(new UnsupportedOperationException(
+            "`explain` request for cursor requests is not supported. "
+            + "Use `explain` for the initial query request."));
+      }
+      // non-explain request
       return queryExecutionFactory.create(request.getCursor().get(), queryListener.get());
     } else {
       // 1.Parse query and convert parse tree (CST) to abstract syntax tree (AST)