ari/pjsip: Make it possible to control transfers through ARI

Introduce a ChannelTransfer event and the ability to notify progress to
ARI. Implement emitting this event from the PJSIP channel instead of
handling the transfer in Asterisk when configured.

Introduce a dialplan function to the PJSIP channel to switch between the
"core" and "ari-only" behavior.

UserNote: Call transfers on the PJSIP channel can now be controlled by
ARI. This can be enabled by using the PJSIP_TRANSFER_HANDLING(ari-only)
dialplan function.

rest-api/api-docs/channels.json

@@ -1997,6 +1997,59 @@
 					]
 				}
 			]
+		},
+		{
+			"path": "/channels/{channelId}/transfer_progress",
+			"description": "Inform the channel that the transfer is in progress.",
+			"operations": [
+				{
+					"httpMethod": "POST",
+					"since": [
+						"22.3.0",
+						"21.8.0",
+						"20.13.0"
+					],
+					"summary": "Inform the channel about the progress of the attended/blind transfer.",
+					"nickname": "transfer_progress",
+					"responseClass": "void",
+					"parameters": [
+						{
+							"name": "channelId",
+							"description": "Channel's id",
+							"paramType": "path",
+							"required": true,
+							"allowMultiple": false,
+							"dataType": "string"
+						},
+						{
+							"name": "states",
+							"description": "The state of the progress",
+							"paramType": "query",
+							"required": true,
+							"allowMultiple": false,
+							"dataType": "string"
+						}
+					],
+					"errorResponses": [
+						{
+							"code": 400,
+							"reason": "Endpoint parameter not provided"
+						},
+						{
+							"code": 404,
+							"reason": "Channel or endpoint not found"
+						},
+						{
+							"code": 409,
+							"reason": "Channel not in a Stasis application"
+						},
+						{
+							"code": 412,
+							"reason": "Channel in invalid state"
+						}
+					]
+				}
+			]
 		}
 	],
 	"models": {