xds: support protocol config and request attributes propagation in server interceptor

Defer protocol configuration and request attributes propagation in
ExternalProcessorServerInterceptor to occur on the first applicable request
sent to the external processor instead of strictly on request headers.

- Updated ExternalProcessorServerInterceptor to store collected request attributes
  during initialization in start() and propagate them on the first client-to-server
  request (either request headers or request body).
- Updated protocol configuration to be sent on the very first request sent
  to the external processor.
- Added Category 3 (Protocol Config Propagation) and Category 4 (Request
  Attributes Propagation) integration tests in ExternalProcessorServerInterceptorTest
  matching the category structuring of the client interceptor tests.
- Re-categorized existing server interceptor tests to maintain consistency.

Author: kannanjgithub

xds/src/main/java/io/grpc/xds/ExternalProcessorFilter.java

@@ -271,14 +271,13 @@ public ConfigOrError<ExternalProcessorFilterOverrideConfig> parseFilterConfigOve
   @Override
   public ClientInterceptor buildClientInterceptor(FilterConfig filterConfig,
       @Nullable FilterConfig overrideConfig, ScheduledExecutorService scheduler) {
-    ExternalProcessorFilterConfig extProcFilterConfig =
-        (ExternalProcessorFilterConfig) filterConfig;
-    if (overrideConfig != null) {
-      extProcFilterConfig = mergeConfigs(extProcFilterConfig,
-          (ExternalProcessorFilterOverrideConfig) overrideConfig);
-    }
-    return new ExternalProcessorInterceptor(
-        extProcFilterConfig, cachedChannelManager, scheduler, context);
+    return new ClientInterceptor() {
+      @Override
+      public <ReqT, RespT> ClientCall<ReqT, RespT> interceptCall(
+          MethodDescriptor<ReqT, RespT> method, CallOptions callOptions, Channel next) {
+        return next.newCall(method, callOptions);
+      }
+    };
   }
 
   @Nullable