Add js-plugins pre-built support

server/jetty/src/main/java/io/deephaven/server/jetty/JettyBackedGrpcServer.java

@@ -94,6 +94,9 @@ public JettyBackedGrpcServer(
         // Wire up the provided grpc filter
         context.addFilter(new FilterHolder(filter), "/*", EnumSet.noneOf(DispatcherType.class));
 
+        // Set up /js-plugins/*
+        JsPlugins.maybeAdd(context);
+
         // Set up websockets for grpc-web - depending on configuration, we can register both in case we encounter a
         // client using "vanilla"
         // grpc-websocket, that can't multiplex all streams on a single socket

server/jetty/src/main/java/io/deephaven/server/jetty/JsPlugins.java

@@ -0,0 +1,29 @@
+package io.deephaven.server.jetty;
+
+import io.deephaven.configuration.Configuration;
+import org.eclipse.jetty.servlet.DefaultServlet;
+import org.eclipse.jetty.servlet.ServletContextHandler;
+import org.eclipse.jetty.servlet.ServletHolder;
+
+class JsPlugins {
+
+    public static void maybeAdd(ServletContextHandler context) {
+        // Note: this would probably be better to live in JettyConfig - but until we establish more formal expectations
+        // for js plugin configuration and workflows, we'll keep this here.
+        final String resourceBase =
+                Configuration.getInstance().getStringWithDefault("deephaven.jsPlugins.resourceBase", null);
+        if (resourceBase == null) {
+            return;
+        }
+        context.addServlet(createServlet("js-plugins", resourceBase), "/js-plugins/*");
+    }
+
+    private static ServletHolder createServlet(String name, String resourceBase) {
+        final ServletHolder jsPlugins = new ServletHolder(name, DefaultServlet.class);
+        jsPlugins.setInitParameter("resourceBase", resourceBase);
+        jsPlugins.setInitParameter("pathInfoOnly", "true");
+        jsPlugins.setInitParameter("dirAllowed", "false");
+        jsPlugins.setAsyncSupported(true);
+        return jsPlugins;
+    }
+}