Add iOS only setLaunchURLsInApp function

[nan-li]

Description

One Line Summary

Adds the setLaunchURLsInApp function that is for iOS only.

Details

The function, previously only available via the native iOS SDK, does the following:

This method can be used to set if launch URLs should be opened in Safari or within the application. The default value is true which opens an in-app browser. Setting this to false will open in Safari.

For Android, just log that it is unsupported and don't add to plugin.

• Add setLaunchURLsInApp method to OneSignal iOS plugin
• Add setLaunchURLsInApp bridge method to onesignal_flutter.dart
• Add setLaunchURLsInApp method to the example app with false

Test

• iOS: Tested on iPhone 13 using the example app calling setLaunchURLsInApp with true and false. Tested with OneSignal-iOS-SDK 3.10.1 that remedied the problem with calling after init.
• Android: Tested on emulator that no error occurred and logged the statement "setLaunchURLsInApp: this function is not supported on Android".
• Did not add unit testing as this method was not added to the Android plugin and the mock channel was not being called as a result, leading to failing unit test.

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This change is 

[jkasten2]

One nit on a log level, feel free to fix and merge without another approval from my end.

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @emawby, @Jeasmine, and @nan-li)

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lib/onesignal_flutter.dart, line 176 at r1 (raw file):

        "OneSignal#setLaunchURLsInApp", {'isEnabled': isEnabled});
    } else {
      _onesignalLog(OSLogLevel.verbose,

nit, I would say this should be info instead of verbose.