[pigeon] Support other hosts in generated file CI checks (flutter#5944)

Reworks Pigeon's CI validation of generated files to support multiple hosts, rather than only Linux, with the ability to set specific languages per host, to allow us to run validation for each language on whatever host it is most convenient to set up a formatter for that language, rather than having to support all languages on Linux.

See discussion in flutter/packages#5928

Part of flutter#41129

Author: stuartmorgan-g

packages/pigeon/example/app/ios/Runner/Messages.g.swift

@@ -34,7 +34,9 @@ private func wrapError(_ error: Any) -> [Any?] {
 }
 
 private func createConnectionError(withChannelName channelName: String) -> FlutterError {
-  return FlutterError(code: "channel-error", message: "Unable to establish connection on channel: '\(channelName)'.", details: "")
+  return FlutterError(
+    code: "channel-error", message: "Unable to establish connection on channel: '\(channelName)'.",
+    details: "")
 }
 
 private func isNullish(_ value: Any?) -> Bool {
@@ -130,7 +132,9 @@ class ExampleHostApiSetup {
   static var codec: FlutterStandardMessageCodec { ExampleHostApiCodec.shared }
   /// Sets up an instance of `ExampleHostApi` to handle messages through the `binaryMessenger`.
   static func setUp(binaryMessenger: FlutterBinaryMessenger, api: ExampleHostApi?) {
-    let getHostLanguageChannel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_example_package.ExampleHostApi.getHostLanguage", binaryMessenger: binaryMessenger, codec: codec)
+    let getHostLanguageChannel = FlutterBasicMessageChannel(
+      name: "dev.flutter.pigeon.pigeon_example_package.ExampleHostApi.getHostLanguage",
+      binaryMessenger: binaryMessenger, codec: codec)
     if let api = api {
       getHostLanguageChannel.setMessageHandler { _, reply in
         do {
@@ -143,7 +147,9 @@ class ExampleHostApiSetup {
     } else {
       getHostLanguageChannel.setMessageHandler(nil)
     }
-    let addChannel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_example_package.ExampleHostApi.add", binaryMessenger: binaryMessenger, codec: codec)
+    let addChannel = FlutterBasicMessageChannel(
+      name: "dev.flutter.pigeon.pigeon_example_package.ExampleHostApi.add",
+      binaryMessenger: binaryMessenger, codec: codec)
     if let api = api {
       addChannel.setMessageHandler { message, reply in
         let args = message as! [Any?]
@@ -159,7 +165,9 @@ class ExampleHostApiSetup {
     } else {
       addChannel.setMessageHandler(nil)
     }
-    let sendMessageChannel = FlutterBasicMessageChannel(name: "dev.flutter.pigeon.pigeon_example_package.ExampleHostApi.sendMessage", binaryMessenger: binaryMessenger, codec: codec)
+    let sendMessageChannel = FlutterBasicMessageChannel(
+      name: "dev.flutter.pigeon.pigeon_example_package.ExampleHostApi.sendMessage",
+      binaryMessenger: binaryMessenger, codec: codec)
     if let api = api {
       sendMessageChannel.setMessageHandler { message, reply in
         let args = message as! [Any?]
@@ -180,15 +188,19 @@ class ExampleHostApiSetup {
 }
 /// Generated protocol from Pigeon that represents Flutter messages that can be called from Swift.
 protocol MessageFlutterApiProtocol {
-  func flutterMethod(aString aStringArg: String?, completion: @escaping (Result<String, FlutterError>) -> Void)
+  func flutterMethod(
+    aString aStringArg: String?, completion: @escaping (Result<String, FlutterError>) -> Void)
 }
 class MessageFlutterApi: MessageFlutterApiProtocol {
   private let binaryMessenger: FlutterBinaryMessenger
   init(binaryMessenger: FlutterBinaryMessenger) {
     self.binaryMessenger = binaryMessenger
   }
-  func flutterMethod(aString aStringArg: String?, completion: @escaping (Result<String, FlutterError>) -> Void) {
-    let channelName: String = "dev.flutter.pigeon.pigeon_example_package.MessageFlutterApi.flutterMethod"
+  func flutterMethod(
+    aString aStringArg: String?, completion: @escaping (Result<String, FlutterError>) -> Void
+  ) {
+    let channelName: String =
+      "dev.flutter.pigeon.pigeon_example_package.MessageFlutterApi.flutterMethod"
     let channel = FlutterBasicMessageChannel(name: channelName, binaryMessenger: binaryMessenger)
     channel.sendMessage([aStringArg] as [Any?]) { response in
       guard let listResponse = response as? [Any?] else {
@@ -201,7 +213,11 @@ class MessageFlutterApi: MessageFlutterApiProtocol {
         let details: String? = nilOrValue(listResponse[2])
         completion(.failure(FlutterError(code: code, message: message, details: details)))
       } else if listResponse[0] == nil {
-        completion(.failure(FlutterError(code: "null-error", message: "Flutter api returned null value for non-null return value.", details: "")))
+        completion(
+          .failure(
+            FlutterError(
+              code: "null-error",
+              message: "Flutter api returned null value for non-null return value.", details: "")))
       } else {
         let result = listResponse[0] as! String
         completion(.success(result))