Updated iOS plugin version to 8.0.0

[Jack37]

This pull request updates the GoogleSignIn plugin for iOS to version 8.0.0
Tested with Firebase 12.8.0

Important note
If you need to request Auth Code, add your Web Client ID from your configuration object to the GoogleService-Info.plist file, under SERVER_CLIENT_ID key.

For example:

GoogleSignIn.Configuration = new GoogleSignInConfiguration
{
    WebClientId = "your-webclient_id.apps.googleusercontent.com",
    RequestAuthCode = true
};

GoogleService-Info.plist

<key>SERVER_CLIENT_ID</key>
<string>your-webclient_id.apps.googleusercontent.com</string>
...

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[hussnainfiaz125]

Thanks @Jack37 your solution worked for me I have been stuck here for weeks