[Feature] Add callbacks to setLanguage

[nan-li]

Description

One Line Summary

Adds callbacks for setting language (setLanguage method).

Details

Motivation

Provides developers with success and failure callbacks.

Scope

• Add setLanguage success and failure callbacks to Android and iOS methods.
• On Android, if multiple channels are set (ie push, email, SMS), the developer's callback will fire once for the first successful or failed channel and subsequent success or failure callbacks will not be invoked.
• On iOS, if multiple channels are set (ie push, email, SMS), the callback will be invoked once with the states of all the channels.
• When testing Android, the onSuccess response from the native SDK is always null. When this is null, I pass through a custom object, because passing through the null object crashes in Flutter.

  {'success' : 'true', 'message' : 'Successfully set language.'}

• Android always passes through a null response, because this line always has a null language.

How to Use

    OneSignal.shared.setLanguage("fr").then((response) { // Example of setting the language to French.
        print("Successfully set language with response: $response");
    }).catchError((error) {
        print("Failed to set language with error: $error");
    });

Testing

Manual testing

Tested setting various languages on iOS and Android.

• ✅ Tested setting a language without using any callbacks.
• ✅ Tested setting a language that accepts the response and catches the failure.
• ✅ Tested setting email and SMS first, then setting a language. See below for the responses the developer receives.

// Android success
{success: true, message: Successfully set language.}

// iOS success
{email: {success: true}, push: {success: true}}

// Android failure example: turn off wifi and data
PlatformException(OneSignal, Encountered an error when setLanguage: Failed to set language., null, null)

// iOS failure example: turn off wifi and data
// Unfortunately, the flutterError does not give much detail.
PlatformException(0, The operation couldn’t be completed. (com.onesignal.language error 0.), null, null)

Affected code checklist

• Notifications
  • Display
  • Open
  • Push Processing
  • Confirm Deliveries
• Outcomes
• Sessions
• In-App Messaging
• REST API requests
• Public API changes

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
• Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• I have included test coverage for these changes, or explained why they are not needed
• All automated tests pass, or I explained why that is not possible
• I have personally tested this on my device, or explained why that is not possible

Final pass

• Code is as readable as possible.
• I have reviewed this PR myself, ensuring it meets each checklist item

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This change is 

[jkasten2]

Reviewed 4 of 4 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @emawby and @tanaynigam)