Open websocket for incoming calls when app is in background

[rikvdbrule]

To enable incoming calls when the application is in the background and smooth CallKit integration, we made some changes within the SparkSDK itself. Our use case is the following:

• Our application registers for PushKit VoIP push notifications as outlined in the Apple documentation about background calls here: https://developer.apple.com/documentation/pushkit
• When someone logs in to the SparkSDK in our app, we register a webhook on incoming calls
• When that person receives a call, the webhook notifies our backend of the incoming call, and we send a VoIP push notification to the person's device.
• We send the onIncomingCallInBackground notification through the NotificationCenter from within the app
• Our modifications in the SparkSDK pick up this notification, and the websocket is opened.
• We are now able to handle the call with the regular flow of the SDK.

We would like to know if either this implementation could be officially supported (by merging this PR or a modified version of it into the main repository), or whether there is a different flow for background calls that we should follow.

[Noble-Six]

Hi @rikvdbrule @buh, if the app wants to fetch incoming call in background mode, we recommend use Phone.register/deregister. Because of WebSocket URL probably expired during the disconnected time. When calling the API register(), SDK will update the WebSocket URL first and then open a new WebSocket. Call the deregister() to disconnect the WebSocket after background task end.