[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/Subcommands/ServerCommand.swift

@@ -0,0 +1,39 @@
+/*
+ * 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 ArgumentParser
+import GRPCCore
+import GRPCNIOTransportHTTP2
+
+struct ServerCommand: AsyncParsableCommand {
+  static let configuration = CommandConfiguration(
+    commandName: "serve")
+
+  func run() async throws {
+    let server = GRPCServer(
+      transport: .http2NIOPosix(
+        address: .ipv4(host: "127.0.0.1", port: port),
+        transportSecurity: .plaintext),
+      services: [GreeterService()])
+
+    try await withThrowingDiscardingTaskGroup { group in
+      group.addTask { try await server.serve() }
+      if let address = try await server.listeningAddress {
+        print("Echo listening on \(address)")
+      }
+    }
+  }
+}