feat: add waitForTappable

[KazuCocoa]

Added but it got below error..., but it is actually https://github.com/flutter/flutter/blob/master/packages/flutter_driver/lib/src/common/deserialization_factory.dart#L59 . Maybe my test app had wrong flutter SDK built one..

[HTTP] --> POST /wd/hub/session/14c867e1-8c43-4ad0-bb11-117a22ca8439/execute/sync
[HTTP] {"script":"flutter:waitForTappable","args":["eyJmaW5kZXJUeXBlIjoiQnlUZXh0IiwidGV4dCI6IlRhcCBtZSEifQ==",1000]}
[debug] [FlutterDriver@7ea1 (14c867e1)] Calling AppiumDriver.execute() with args: ["flutter:waitForTappable",["eyJmaW5kZXJUeXBlIjoiQnlUZXh0IiwidGV4dCI6IlRhcCBtZSEifQ==",1000],"14c867e1-8c43-4ad0-bb11-117a22ca8439"]
[debug] [FlutterDriver] Executing Flutter driver command 'execute'
[debug] [FlutterDriver] >>> {"command":"waitForTappable","finderType":"ByText","text":"Tap me!","timeout":1000000}
[debug] [FlutterDriver] <<< {"isError":true,"response":"Uncaught extension error while executing waitForTappable: DriverError: Unsupported command kind waitForTappable\n\n#0      DeserializeCommandFactory.deserializeCommand (package:flutter_driver/src/common/deserialization_factory.dart:68:5)\n#1      FlutterDriverExtension.deserializeCommand (package:flutter_driver/src/extension/extension.dart:425:18)\n#2      FlutterDriverExtension.call (package:flutter_driver/src/extension/extension.dart:371:31)\n#3      BindingBase.registerServiceExtension.<anonymous closure> (package:flutter/src/foundation/binding.dart:597:32)\n<asynchronous suspension>\n","type":"_extensionType","method":"ext.flutter.driver"} | previous command waitForTappable
[debug] [FlutterDriver@7ea1 (14c867e1)] Encountered internal error running command: Error: Cannot execute command waitForTappable, server reponse {
[debug] [FlutterDriver@7ea1 (14c867e1)]   "isError": true,
[debug] [FlutterDriver@7ea1 (14c867e1)]   "response": "Uncaught extension error while executing waitForTappable: DriverError: Unsupported command kind waitForTappable\n\n#0      DeserializeCommandFactory.deserializeCommand (package:flutter_driver/src/common/deserialization_factory.dart:68:5)\n#1      FlutterDriverExtension.deserializeCommand (package:flutter_driver/src/extension/extension.dart:425:18)\n#2      FlutterDriverExtension.call (package:flutter_driver/src/extension/extension.dart:371:31)\n#3      BindingBase.registerServiceExtension.<anonymous closure> (package:flutter/src/foundation/binding.dart:597:32)\n<asynchronous suspension>\n",
[debug] [FlutterDriver@7ea1 (14c867e1)]   "type": "_extensionType",
[debug] [FlutterDriver@7ea1 (14c867e1)]   "method": "ext.flutter.driver"
[debug] [FlutterDriver@7ea1 (14c867e1)] }
[debug] [FlutterDriver@7ea1 (14c867e1)]     at FlutterDriver.executeElementCommand (/Users/kazuaki/GitHub/appium-flutter-driver/driver/lib/sessions/observatory.ts:151:11)
[debug] [FlutterDriver@7ea1 (14c867e1)]     at runMicrotasks (<anonymous>)
[debug] [FlutterDriver@7ea1 (14c867e1)]     at processTicksAndRejections (internal/process/task_queues.js:95:5)
[debug] [FlutterDriver@7ea1 (14c867e1)]     at /Users/kazuaki/GitHub/appium-flutter-driver/driver/lib/commands/execute/wait.ts:20:3
[HTTP] <-- POST /wd/hub/session/14c867e1-8c43-4ad0-bb11-117a22ca8439/execute/sync 500 1513 ms - 2109

[KazuCocoa]

added this in 1.2.0, hope this will work in a proper flutter version...