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

res/ari/ari_model_validators.h

@@ -1432,6 +1432,7 @@ ari_validator ast_ari_validate_application_fn(void);
  * Channel
  * - accountcode: string (required)
  * - caller: CallerID (required)
+ * - channelvars: object
  * - connected: CallerID (required)
  * - creationtime: Date (required)
  * - dialplan: DialplanCEP (required)