ARI/PJSIP: Add the ability to redirect (transfer) a channel in a Stasis app

This patch adds a new feature to ARI to redirect a channel to another server,
and fixes a few bugs in PJSIP's handling of the Transfer dialplan
application/ARI redirect capability.

*New Feature*
A new operation has been added to the ARI channels resource, redirect. With
this, a channel in a Stasis application can be redirected to another endpoint
of the same underlying channel technology.

*Bug fixes*
In the process of writing this new feature, two bugs were fixed in the PJSIP
stack:
(1) The existing .transfer channel callback had the limitation that it could
    only transfer channels to a SIP URI, i.e., you had to pass
    'PJSIP/sip:foo@my_provider.com' to the dialplan application. While this is
    still supported, it is somewhat unintuitive - particularly in a world full
    of endpoints. As such, we now also support specifying the PJSIP endpoint to
    transfer to.
(2) res_pjsip_multihomed was, unfortunately, trying to 'help' a 302 redirect by
    updating its Contact header. Alas, that resulted in the forwarding
    destination set by the dialplan application/ARI resource/whatever being
    rewritten with very incorrect information. Hence, we now don't bother
    updating an outgoing response if it is a 302. Since this took a looong time
    to find, some additional debug statements have been added to those modules
    that update the Contact headers.

Review: https://reviewboard.asterisk.org/r/4316/

ASTERISK-24015 #close
Reported by: Private Name

ASTERISK-24703 #close
Reported by: Matt Jordan



git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@431717 65c4cc65-6c06-0410-ace0-fbb531ad65f3

rest-api/api-docs/channels.json

@@ -396,6 +396,54 @@
 				}
 			]
 		},
+		{
+			"path": "/channels/{channelId}/redirect",
+			"description": "Inform the channel that it should redirect itself to a different location. Note that this will almost certainly cause the channel to exit the application.",
+			"operations": [
+				{
+					"httpMethod": "POST",
+					"summary": "Redirect the channel to a different location.",
+					"nickname": "redirect",
+					"responseClass": "void",
+					"parameters": [
+						{
+							"name": "channelId",
+							"description": "Channel's id",
+							"paramType": "path",
+							"required": true,
+							"allowMultiple": false,
+							"dataType": "string"
+						},
+						{
+							"name": "endpoint",
+							"description": "The endpoint to redirect the channel to",
+							"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": 422,
+							"reason": "Endpoint is not the same type as the channel"
+						}
+					]
+				}
+			]
+		},
 		{
 			"path": "/channels/{channelId}/answer",
 			"description": "Answer a channel",