Improvements to gRPC server-side support (#8765)

* Cleanup of TODOs in GrpcProtocolHandler. New utility class for converting headers.

* New implementation of GrpcHeadersUtil without using any internal APIs.

* Initial support for compression/decompression of gRPC data.

* Adds support for codec negotiation and new testing showing gZIP compression support in gRPC calls.

* Uses a client interceptor to verify that response is properly encoded using GZIP.

* Fixes import problems and re-arranges private fields.

webserver/grpc/src/main/java/io/helidon/webserver/grpc/GrpcHeadersUtil.java

@@ -0,0 +1,60 @@
+/*
+ * Copyright (c) 2024 Oracle and/or its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.helidon.webserver.grpc;
+
+import java.util.Base64;
+
+import io.helidon.http.HeaderNames;
+import io.helidon.http.WritableHeaders;
+import io.helidon.http.http2.Http2Headers;
+
+import io.grpc.Metadata;
+
+import static java.nio.charset.StandardCharsets.US_ASCII;
+
+class GrpcHeadersUtil {
+
+    private GrpcHeadersUtil() {
+    }
+
+    static void updateHeaders(WritableHeaders<?> writable, Metadata headers) {
+        Base64.Encoder encoder = Base64.getEncoder();
+        headers.keys().forEach(name -> {
+            if (name.endsWith(Metadata.BINARY_HEADER_SUFFIX)) {
+                byte[] binary = headers.get(Metadata.Key.of(name, Metadata.BINARY_BYTE_MARSHALLER));
+                writable.add(HeaderNames.create(name, new String(encoder.encode(binary), US_ASCII)));
+            } else {
+                String ascii = headers.get(Metadata.Key.of(name, Metadata.ASCII_STRING_MARSHALLER));
+                writable.add(HeaderNames.create(name, ascii));
+            }
+        });
+    }
+
+    static Metadata toMetadata(Http2Headers headers) {
+        Base64.Decoder decoder = Base64.getDecoder();
+        Metadata metadata = new Metadata();
+        headers.httpHeaders().forEach(header -> {
+            String name = header.name();
+            if (name.endsWith(Metadata.BINARY_HEADER_SUFFIX)) {
+                metadata.put(Metadata.Key.of(name, Metadata.BINARY_BYTE_MARSHALLER),
+                             decoder.decode(name.getBytes(US_ASCII)));
+            } else {
+                metadata.put(Metadata.Key.of(name, Metadata.ASCII_STRING_MARSHALLER), name);
+            }
+        });
+        return metadata;
+    }
+}