ARI: REST over Websocket

ARI: REST over Websocket

This commit adds the ability to make ARI REST requests over the same
websocket used to receive events.

For full details on how to use the new capability, visit...

https://docs.asterisk.org/Configuration/Interfaces/Asterisk-REST-Interface-ARI/ARI-REST-over-WebSocket/

Changes:

* Added utilities to http.c:
  * ast_get_http_method_from_string().
  * ast_http_parse_post_form().
* Added utilities to json.c:
  * ast_json_nvp_array_to_ast_variables().
  * ast_variables_to_json_nvp_array().
* Added definitions for new events to carry REST responses.
* Created res/ari/ari_websocket_requests.c to house the new request handlers.
* Moved non-event specific code out of res/ari/resource_events.c into
  res/ari/ari_websockets.c
* Refactored res/res_ari.c to move non-http code out of ast_ari_callback()
  (which is http specific) and into ast_ari_invoke() so it can be shared
  between both the http and websocket transports.

UpgradeNote: This commit adds the ability to make ARI REST requests over the same
websocket used to receive events.
See https://docs.asterisk.org/Configuration/Interfaces/Asterisk-REST-Interface-ARI/ARI-REST-over-WebSocket/

(cherry picked from commit 1442c171417bd5cb1373a8ae067777277df519e1)