res_stasis: Expose event for call forwarding and follow forwarded channel.

This change adds an event for when an originated call is redirected to
another target. This event contains the original channel and the newly
created channel. If a stasis subscription exists on the original originated
channel for a stasis application then a new subscription will also be
created on the stasis application to the redirected channel. This allows
the application to follow the call path completely.

(closes issue ASTERISK-22719)
Reported by: Joshua Colp

Review: https://reviewboard.asterisk.org/r/3054/
........

Merged revisions 403808 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@403810 65c4cc65-6c06-0410-ace0-fbb531ad65f3

rest-api/api-docs/events.json

@@ -98,6 +98,7 @@
 				"ChannelHangupRequest",
 				"ChannelVarset",
 				"EndpointStateChange",
+				"Dial",
 				"StasisEnd",
 				"StasisStart"
 			]
@@ -411,6 +412,42 @@
 				}
 			}
 		},
+		"Dial": {
+			"id": "Dial",
+			"description": "Dialing state has changed.",
+			"properties": {
+				"caller": {
+					"required": false,
+					"type": "Channel",
+					"description": "The calling channel."
+				},
+				"peer": {
+					"required": true,
+					"type": "Channel",
+					"description": "The dialed channel."
+				},
+				"forward": {
+					"required": false,
+					"type": "string",
+					"description": "Forwarding target requested by the original dialed channel."
+				},
+				"forwarded": {
+					"required": false,
+					"type": "Channel",
+					"description": "Channel that the caller has been forwarded to."
+				},
+				"dialstring": {
+					"required": false,
+					"type": "string",
+					"description": "The dial string for calling the peer channel."
+				},
+				"dialstatus": {
+					"required": true,
+					"type": "string",
+					"description": "Current status of the dialing attempt to the peer."
+				}
+			}
+		},
 		"StasisEnd": {
 			"id": "StasisEnd",
 			"description": "Notification that a channel has left a Stasis application.",