[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/server/main.swift

@@ -1,97 +0,0 @@
-/*
- * 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 GRPC
-import Logging
-import Model
-import NIO
-
-class Greeter: models_GreeterProvider {
-
-  var interceptors: models_GreeterServerInterceptorFactoryProtocol?
-
-  let greetings: [String]
-
-  init() {
-    greetings = ["Hi", "Hallo", "Ciao"]
-  }
-
-  func SayHello(
-    request: Message<models_HelloRequest>,
-    context: StatusOnlyCallContext)
-    -> EventLoopFuture<Message<models_HelloReply>>
-  {
-    let recipient = request.object.name ?? "Stranger"
-
-    var builder = FlatBufferBuilder()
-    let off = builder.create(string: "Hello \(recipient)")
-    let root = models_HelloReply.createHelloReply(&builder, messageOffset: off)
-    builder.finish(offset: root)
-    return context.eventLoop
-      .makeSucceededFuture(Message<models_HelloReply>(builder: &builder))
-  }
-
-  func SayManyHellos(
-    request: Message<models_HelloRequest>,
-    context: StreamingResponseCallContext<Message<models_HelloReply>>)
-    -> EventLoopFuture<GRPCStatus>
-  {
-    for name in greetings {
-      var builder = FlatBufferBuilder()
-      let off =
-        builder
-          .create(string: "\(name) \(request.object.name ?? "Unknown")")
-      let root = models_HelloReply.createHelloReply(
-        &builder,
-        messageOffset: off)
-      builder.finish(offset: root)
-      _ = context.sendResponse(Message<models_HelloReply>(builder: &builder))
-    }
-    return context.eventLoop.makeSucceededFuture(.ok)
-  }
-}
-
-// Quieten the logs.
-LoggingSystem.bootstrap {
-  var handler = StreamLogHandler.standardOutput(label: $0)
-  handler.logLevel = .critical
-  return handler
-}
-
-let group = MultiThreadedEventLoopGroup(numberOfThreads: 1)
-defer {
-  try! group.syncShutdownGracefully()
-}
-
-// Create some configuration for the server:
-let configuration = Server.Configuration(
-  target: .hostAndPort("localhost", 0),
-  eventLoopGroup: group,
-  serviceProviders: [Greeter()])
-
-// Start the server and print its address once it has started.
-let server = Server.start(configuration: configuration)
-server.map {
-  $0.channel.localAddress
-}.whenSuccess { address in
-  print("server started on port \(address!.port!)")
-}
-
-// Wait on the server's `onClose` future to stop the program from exiting.
-_ = try server.flatMap {
-  $0.onClose
-}.wait()