[Swift] Migrate to swift 6.0 and Implements support gRPC v2 (#8983)

* Migrate to swift 6.0 & swift-gRPC 2.0

The following migrates to swift 6.0, and also
migrate to swift-grpc 2.0 that uses swift-nio
under the hood to provide nicer API and async await

Adds sendable to enum & update @_implementationOnly imports to use internal imports

* Address PR comments regarding misspelling & proper method naming.

grpc/examples/swift/Greeter/Sources/Commands/Services/GreeterService.swift

@@ -0,0 +1,121 @@
+/*
+ * Copyright 2024 Google Inc. 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 FlatBuffers
+import GRPCCore
+import Models
+
+struct GreeterService: models_Greeter.SimpleServiceProtocol {
+  func Get(
+    request: GRPCMessage<models_HelloResponse>,
+    context: GRPCCore
+      .ServerContext) async throws -> GRPCMessage<models_HelloResponse>
+  {
+    let model = try request.decode()
+    print("## GreeterService.Get: \(model.message)")
+
+    var builder = FlatBufferBuilder(initialSize: 128)
+    let off = builder.create(string: "Hello \(model.message ?? "Unknown")")
+    let root = models_HelloResponse.createHelloResponse(
+      &builder,
+      messageOffset: off)
+    builder.finish(offset: root)
+    return try GRPCMessage<models_HelloResponse>(builder: &builder)
+  }
+
+  func Collect(
+    request: GRPCCore.RPCAsyncSequence<
+      GRPCMessage<models_HelloResponse>,
+      any Swift.Error
+    >,
+    context: GRPCCore
+      .ServerContext) async throws -> GRPCMessage<models_HelloResponse>
+  {
+    let messages: [String] = try await request
+      .reduce(into: []) { array, message in
+        let model = try message.decode()
+        return array.append(model.message ?? "Unknown")
+      }
+
+    let joined = messages.joined(separator: ", ")
+    print("## GreeterService.Collect: \(joined)")
+
+    var builder = FlatBufferBuilder(initialSize: 128)
+    let off = builder.create(string: "Hello \(joined)")
+    let root = models_HelloResponse.createHelloResponse(
+      &builder,
+      messageOffset: off)
+    builder.finish(offset: root)
+    return try GRPCMessage<models_HelloResponse>(builder: &builder)
+  }
+
+  func Expand(
+    request: GRPCMessage<models_HelloResponse>,
+    response: GRPCCore.RPCWriter<GRPCMessage<models_HelloResponse>>,
+    context: GRPCCore.ServerContext) async throws
+  {
+
+    let model = try request.decode()
+    print("## GreeterService.Expand: \(model.message)")
+
+    let message = model.message ?? "Unknown"
+    let stream = AsyncThrowingStream<
+      GRPCMessage<models_HelloResponse>,
+      Error
+    > { continuation in
+      var builder = FlatBufferBuilder(initialSize: 128)
+      for char in message {
+        let off = builder.create(string: "\(char)")
+        let root = models_HelloResponse.createHelloResponse(
+          &builder,
+          messageOffset: off)
+        builder.finish(offset: root)
+
+        do {
+          continuation
+            .yield(try GRPCMessage<models_HelloResponse>(builder: &builder))
+        } catch {
+          continuation.finish(throwing: error)
+        }
+      }
+
+      continuation.finish()
+    }
+    try await response.write(contentsOf: stream)
+  }
+
+  func Update(
+    request: GRPCCore.RPCAsyncSequence<
+      GRPCMessage<models_HelloResponse>,
+      any Swift.Error
+    >,
+    response: GRPCCore.RPCWriter<GRPCMessage<models_HelloResponse>>,
+    context: GRPCCore.ServerContext) async throws
+  {
+    for try await message in request {
+      let model = try message.decode()
+      print("## GreeterService.Update: \(model.message)")
+      var builder = FlatBufferBuilder(initialSize: 128)
+      let off = builder.create(string: "Hello \(model.message ?? "Unknown")")
+      let root = models_HelloResponse.createHelloResponse(
+        &builder,
+        messageOffset: off)
+      builder.finish(offset: root)
+      try await response
+        .write(try GRPCMessage<models_HelloResponse>(builder: &builder))
+    }
+  }
+}