Add generator meta into head

application/src/main/java/run/halo/app/theme/dialect/MetaGeneratorProcessor.java

@@ -0,0 +1,35 @@
+package run.halo.app.theme.dialect;
+
+import org.springframework.beans.factory.ObjectProvider;
+import org.springframework.boot.info.BuildProperties;
+import org.springframework.core.annotation.Order;
+import org.springframework.stereotype.Component;
+import org.thymeleaf.context.ITemplateContext;
+import org.thymeleaf.model.IModel;
+import org.thymeleaf.processor.element.IElementModelStructureHandler;
+import reactor.core.publisher.Mono;
+
+@Order()
+@Component
+public class MetaGeneratorProcessor implements TemplateHeadProcessor {
+
+    private final String generatorValue;
+
+    public MetaGeneratorProcessor(ObjectProvider<BuildProperties> buildProperties) {
+        this.generatorValue = "Halo " + buildProperties.stream().findFirst()
+            .map(BuildProperties::getVersion)
+            .orElse("Unknown");
+    }
+
+    @Override
+    public Mono<Void> process(ITemplateContext context, IModel model,
+        IElementModelStructureHandler structureHandler) {
+        return Mono.fromRunnable(() -> {
+            var modelFactory = context.getModelFactory();
+            var generatorMeta =
+                modelFactory.createStandaloneElementTag("meta", "generator", generatorValue);
+            model.add(generatorMeta);
+        });
+    }
+
+}