CallKit: Support initiating outgoing phone calls

[mwild1]

Describe your feature proposal
When any of the enabled accounts have a telephony provider configured, Snikket should inform iOS that it is able to handle phone numbers via supportedHandleTypes of CXProviderConfiguration, and allow the user to call contacts from their address book using Snikket (via long press on the 'call' button).

• If a single account has a telephony provider configured, outgoing calls should always use that account
• If multiple accounts have a telephony provider configured, show an action sheet to select the account to call with

Is your feature request related to a problem? Please describe.
This feature allows users with existing contacts in their address book to call them via an XMPP telephony gateway without granting Snikket access to their phone address book.

[mwild1]

Advertising .phoneNumber as a supported handle type is not all that's required, the app needs to handle "start call" intents.

iOS allows apps to handle a "start call" intent, which can be used to launch the app to make a call from the native phone app's "Recents" list, contact cards, Siri, and shortcuts.

Resources

• Enhancing VoIP Apps with CallKit - Apple WWDC 2016
• Test phone numbers - you can call these phone numbers for test purposes