Expose sendInfo from RTCSession to Call.sendInfo

[makstheimba]

Fix for #185

[cloudwebrtc]

Cool, Maybe we should move InfoEvent from internal events to the public?
https://github.com/flutter-webrtc/dart-sip-ua/blob/master/lib/src/event_manager/internal_events.dart#L74

like info_events.dart and provide InfoEvent listener where.
https://github.com/flutter-webrtc/dart-sip-ua/blob/master/lib/src/sip_ua_helper.dart#L182

[makstheimba]

@cloudwebrtc Do I understand correctly that in your comment you propose to add an ability to also listening for InfoEvents?

It may be a good suggestion. We didn't need it in our project and because of that I didn't thought about adding it 🤔

[cloudwebrtc]

Okay，When someone needs this feature, I will create a new PR.