res_ari: Add support for channel variables in ARI events.

This works the same as for AMI manager variables. Set
"channelvars=foo,bar" in your ari.conf general section, and then the
channel variables "foo" and "bar" (along with their values), will
appear in every Stasis websocket channel event.

ASTERISK-26492 #close
patches:
  ari_vars.diff submitted by Mark Michelson

Change-Id: I5609ba239259577c0948645df776d7f3bc864229

rest-api/api-docs/channels.json

@@ -1761,6 +1761,11 @@
 					"required": true,
 					"type": "string",
 					"description": "The default spoken language"
+				},
+				"channelvars": {
+					"required": false,
+					"type": "object",
+					"description": "Channel variables"
 				}
 			}
 		}