Add back GRPCPayload support for server handlers

Motivation:

Recent work in grpc#886 and grpc#889 made `GRPCProtobufPayload` redundant. Since
we broke this work into multiple PRs we temporarily dropped support for
custom `GRPCPayload`s on the server. This PR adds that back.

Modifications:

- Add `GRPCPayload` support back to the server by adding the relevant
  call handler factory functions
- Re-enable the custom payload tests
- Add a few more custom payload tests (since they were limited to just
  bidirectional streaming)

Result:

- Clients and servers support `SwiftProtobuf.Message` and `GRPCPayload`
  separately without using `GRPCProtobufPayload` to bridge between them.

Sources/GRPC/CallHandlers/CallHandlerFactory.swift

@@ -1,18 +1,18 @@
 /*
-* Copyright 2020, gRPC Authors All rights reserved.
-*
-* 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.
-*/
+ * Copyright 2020, gRPC Authors All rights reserved.
+ *
+ * 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.
+ */
 import NIO
 import SwiftProtobuf
 
@@ -34,6 +34,18 @@ public enum CallHandlerFactory {
     )
   }
 
+  public static func makeUnary<Request: GRPCPayload, Response: GRPCPayload>(
+    callHandlerContext: CallHandlerContext,
+    eventObserverFactory: @escaping (UnaryContext<Response>) -> UnaryEventObserver<Request, Response>
+  ) -> UnaryCallHandler<Request, Response> {
+    return UnaryCallHandler(
+      serializer: GRPCPayloadSerializer(),
+      deserializer: GRPCPayloadDeserializer(),
+      callHandlerContext: callHandlerContext,
+      eventObserverFactory: eventObserverFactory
+    )
+  }
+
   public typealias ClientStreamingContext<Response> = UnaryResponseCallContext<Response>
   public typealias ClientStreamingEventObserver<Request> = EventLoopFuture<(StreamEvent<Request>) -> Void>
 
@@ -49,6 +61,18 @@ public enum CallHandlerFactory {
     )
   }
 
+  public static func makeClientStreaming<Request: GRPCPayload, Response: GRPCPayload>(
+    callHandlerContext: CallHandlerContext,
+    eventObserverFactory: @escaping (ClientStreamingContext<Response>) -> ClientStreamingEventObserver<Request>
+  ) -> ClientStreamingCallHandler<Request, Response> {
+    return ClientStreamingCallHandler(
+      serializer: GRPCPayloadSerializer(),
+      deserializer: GRPCPayloadDeserializer(),
+      callHandlerContext: callHandlerContext,
+      eventObserverFactory: eventObserverFactory
+    )
+  }
+
   public typealias ServerStreamingContext<Response> = StreamingResponseCallContext<Response>
   public typealias ServerStreamingEventObserver<Request> = (Request) -> EventLoopFuture<GRPCStatus>
 
@@ -64,6 +88,18 @@ public enum CallHandlerFactory {
     )
   }
 
+  public static func makeServerStreaming<Request: GRPCPayload, Response: GRPCPayload>(
+    callHandlerContext: CallHandlerContext,
+    eventObserverFactory: @escaping (ServerStreamingContext<Response>) -> ServerStreamingEventObserver<Request>
+  ) -> ServerStreamingCallHandler<Request, Response> {
+    return ServerStreamingCallHandler(
+      serializer: GRPCPayloadSerializer(),
+      deserializer: GRPCPayloadDeserializer(),
+      callHandlerContext: callHandlerContext,
+      eventObserverFactory: eventObserverFactory
+    )
+  }
+
   public typealias BidirectionalStreamingContext<Response> = StreamingResponseCallContext<Response>
   public typealias BidirectionalStreamingEventObserver<Request> = EventLoopFuture<(StreamEvent<Request>) -> Void>
 
@@ -78,4 +114,16 @@ public enum CallHandlerFactory {
       eventObserverFactory: eventObserverFactory
     )
   }
+
+  public static func makeBidirectionalStreaming<Request: GRPCPayload, Response: GRPCPayload>(
+    callHandlerContext: CallHandlerContext,
+    eventObserverFactory: @escaping (BidirectionalStreamingContext<Response>) -> BidirectionalStreamingEventObserver<Request>
+  ) -> BidirectionalStreamingCallHandler<Request, Response> {
+    return BidirectionalStreamingCallHandler(
+      serializer: GRPCPayloadSerializer(),
+      deserializer: GRPCPayloadDeserializer(),
+      callHandlerContext: callHandlerContext,
+      eventObserverFactory: eventObserverFactory
+    )
+  }
 }