ARI: Add the ability to intercept hold and raise an event

For some applications - such as SLA - a phone pressing hold should not behave
in the fashion that the Asterisk core would like it to. Instead, the hold
action has some application specific behaviour associated with it - such as
disconnecting the channel that initiated the hold; only playing MoH to channels
in the bridge if the channels are of a particular type, etc.

One way of accomplishing this is to use a framehook to intercept the
hold/unhold frames, raise an event, and eat the frame. Tasty. This patch
accomplishes that using a new dialplan function, HOLD_INTERCEPT.

In addition, some general cleanup of raising hold/unhold Stasis messages was
done, including removing some RAII_VAR usage.

Review: https://reviewboard.asterisk.org/r/4549/

ASTERISK-24922 #close


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@434216 65c4cc65-6c06-0410-ace0-fbb531ad65f3

rest-api/api-docs/events.json

@@ -161,6 +161,8 @@
 				"ChannelVarset",
 				"ChannelTalkingStarted",
 				"ChannelTalkingFinished",
+				"ChannelHold",
+				"ChannelUnhold",
 				"EndpointStateChange",
 				"Dial",
 				"StasisEnd",
@@ -598,6 +600,28 @@
 				}
 			}
 		},
+		"ChannelHold": {
+			"id": "ChannelHold",
+			"description": "A channel initiated a media hold.",
+			"properties": {
+				"channel": {
+					"required": true,
+					"type": "Channel",
+					"description": "The channel that initiated the hold event."
+				}
+			}
+		},
+		"ChannelUnhold": {
+			"id": "ChannelUnhold",
+			"description": "A channel initiated a media unhold.",
+			"properties": {
+				"channel": {
+					"required": true,
+					"type": "Channel",
+					"description": "The channel that initiated the unhold event."
+				}
+			}
+		},
 		"ChannelTalkingStarted": {
 			"id": "ChannelTalkingStarted",
 			"description": "Talking was detected on the channel.",