Refactor sip method and some bug fixes

[rlsutton1]

This pull request refactors the Sip Method from a String to an Enum and infers types where possible which should ease the long term maintenance of the project.

Included are a few bug fixes, namely

• the use of Slice() is SIPMessage.dart
• fix for Exception in Parser.dart

[cloudwebrtc]

There was an error when compiling.

The getter 'signalingState' isn't defined for the class 'RTCPeerConnection'.

Compiler message:                                                       
../lib/src/RTCSession.dart:2268:55: Error: The getter 'signalingState' isn't defined for the class 'RTCPeerConnection'.
 - 'RTCPeerConnection' is from 'package:flutter_webrtc/rtc_peerconnection.dart' ('../../../../.pub-cache/git/flutter-webrtc-dd6edbede69d7f83fd277c250ae4b9e61f07f374/lib/rtc_peerconnection.dart').
Try correcting the name to the name of an existing getter, or defining a getter or field named 'signalingState'.
      debugerror('current state is ${this._connection.signalingState}');
                                                      ^^^^^^^^^^^^^^    
Compiler failed on /Users/weiweiduan/Desktop/Test/dart-sip-ua/example/lib/main.dart
                                                                        
FAILURE: Build failed with an exception.

[cloudwebrtc]

Using a strong type instead of a var variable declaration is a very good idea, and later will gradually replace the var declaration with a real type.

[rlsutton1]

I've removed error, It should compile cleanly now

[cloudwebrtc]

@rlsutton1 I have merged the PR and regenerated the grammar parser.
on

//TODO: Commented out because this seems to cause errors with Asterisk

//await this._connection.setRemoteDescription(answer);

I temporarily removed the comments, I think the process here should be no problem, the error should be caused by other reasons.

You can provide some device logs about asterisk calls to help locate the problem.

[rlsutton1]

@rlsutton1 I have merged the PR and regenerated the grammar parser.
on

//TODO: Commented out because this seems to cause errors with Asterisk

//await this._connection.setRemoteDescription(answer);

I temporarily removed the comments, I think the process here should be no problem, the error should be caused by other reasons.

You can provide some device logs about asterisk calls to help locate the problem.

OK, I'll try to provide some logs tomorrow.