Make WebSocketService interface (line#5368)

Motivation:

`WebsocketService` is a final class, so it is not easy to make another
service run WebSocket and HTTP requests on the same path.

`WebSocketService` is changed as a marker interface in line#5280. However,
the interface does not have API, the service cannot take advantage of
the protocol detection of the original `WebSocketService`. Graphql
WebSocket implementation copied and pasted similar code for WebSocket
detection.

The design of the code here was determined after discussions with other
maintainers based on the idea from
line#5280 (comment)

Modifications:

- Add `WebSocketService` interface and rename the original one to
`DefaultWebSocketService`
- Add `WebSocketProtocolHandler` that is in charge of:
  - Upgrade HTTP request to WebSocket
  - Decode HTTP request to WebSocket for inbound
  - Encode WebSocket to HTTP response for outbound
- Refactor `DefaultWebSocketService` to implement new methods
`WebSocketService` and `WebSocketProtocolHandler`
- Modify Webservicebuilder so that a fallback service can be configured.
- The fallback service would be useful when WebSocket and HTTP request
is served at the same path.

Result:

- You can now set a fallback service to `WebSocketService` via
`WebSocketServiceBuilder.fallbackService()`
- Refactor `WebsocketService` to enable composion and delegatation.

core/src/main/java/com/linecorp/armeria/client/websocket/WebSocketClientFrameDecoder.java

@@ -25,7 +25,7 @@ final class WebSocketClientFrameDecoder extends WebSocketFrameDecoder {
 
     WebSocketClientFrameDecoder(ClientRequestContext ctx, int maxFramePayloadLength,
                                 boolean allowMaskMismatch) {
-        super(ctx, maxFramePayloadLength, allowMaskMismatch);
+        super(maxFramePayloadLength, allowMaskMismatch);
         this.ctx = ctx;
     }
 

core/src/main/java/com/linecorp/armeria/common/websocket/WebSocket.java

@@ -17,6 +17,7 @@
 
 import com.linecorp.armeria.common.annotation.UnstableApi;
 import com.linecorp.armeria.common.stream.StreamMessage;
+import com.linecorp.armeria.internal.common.websocket.WebSocketWrapper;
 
 /**
  * A {@link StreamMessage} that publishes {@link WebSocketFrame}s.
@@ -30,4 +31,12 @@ public interface WebSocket extends StreamMessage<WebSocketFrame> {
     static WebSocketWriter streaming() {
         return new DefaultWebSocket();
     }
+
+    /**
+     * Returns a new {@link WebSocket} whose stream is produced from the specified {@link StreamMessage}.
+     */
+    @UnstableApi
+    static WebSocket of(StreamMessage<WebSocketFrame> delegate) {
+        return new WebSocketWrapper(delegate);
+    }
 }

core/src/main/java/com/linecorp/armeria/internal/common/websocket/WebSocketFrameDecoder.java

@@ -34,7 +34,6 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-import com.linecorp.armeria.common.RequestContext;
 import com.linecorp.armeria.common.annotation.Nullable;
 import com.linecorp.armeria.common.stream.HttpDecoder;
 import com.linecorp.armeria.common.stream.StreamDecoderInput;
@@ -64,7 +63,6 @@ enum State {
         CORRUPT
     }
 
-    private final RequestContext ctx;
     private final int maxFramePayloadLength;
     private final boolean allowMaskMismatch;
     @Nullable
@@ -81,8 +79,7 @@ enum State {
     private boolean receivedClosingHandshake;
     private State state = State.READING_FIRST;
 
-    protected WebSocketFrameDecoder(RequestContext ctx, int maxFramePayloadLength, boolean allowMaskMismatch) {
-        this.ctx = ctx;
+    protected WebSocketFrameDecoder(int maxFramePayloadLength, boolean allowMaskMismatch) {
         this.maxFramePayloadLength = maxFramePayloadLength;
         this.allowMaskMismatch = allowMaskMismatch;
     }