fix for recent Unity iOS and google signin libraries

[umegaya]

purpose

• I was trying to use this project with play-services-resolver-1.2.124.0 and found:
  • opened with recent Unity Editor, build was failed
  • for iOS, generated xcode project is broken.
• this PR aims to:
  • fix various problem to make this project work with recent Unity and google signin libraries
  • make the plugin be easier for user to integrate their OAuth setting (docs not so clear and not automated with PostProcessBuild)

fixes

• removes Unity.Tasks.dll because recent Unity has built in support for it
• removes missing symbols of GIDSignin (function, error code)
• swap preStartUnity instead of application:didFinishLaunchingWithOptions: of UnityAppController with GoogleSignInController version. because recent GIDSignin required to set presentingViewController to work, but correct UIViewController cannot be retrieved on application:didFinishLaunchingWithOptions being called.
• add post processor to make generated Unity Xcode project be able to run with user's OAuth credentials settings. it does:
  • find file which name is GoogleService-Info.plist and copy it to root of xcode project dir
  • read above file and set value of REVERSED_CLIENT_ID as custom URL Type
• added GoogleSignIn_CanBeSilent to C linkage codes

question

• various type of GIDSignin handleURL is missing. how do I handle these with recent lib?