ARI: Add the ability to play multiple media URIs in a single operation

Many ARI applications will want to play multiple media files in a row to
a resource. The most common use case is when building long-ish IVR prompts
made up of multiple, smaller sound files. Today, that requires building a
small state machine, listening for each PlaybackFinished event, and triggering
the next sound file to play. While not especially challenging, it is tedious
work. Since requiring developers to write tedious code to do normal activities
stinks, this patch adds the ability to play back a list of media files to a
resource.

Each of the 'play' operations on supported resources (channels and bridges)
now accepts a comma delineated list of media URIs to play. A single Playback
resource is created as a handle to the entire list. The operation of playing
a list is identical to playing a single media URI, save that a new event,
PlaybackContinuing, is raised instead of a PlaybackFinished for each non-final
media URI. When the entire list is finished being played, a PlaybackFinished
event is raised.

In order to help inform applications where they are in the list playback, the
Playback resource now includes a new, optional attribute, 'next_media_uri',
that contains the next URI in the list to be played.

It's important to note the following:
 - If an offset is provided to the 'play' operations, it only applies to the
   first media URI, as it would be weird to skip n seconds forward in every
   media resource.
 - Operations that control the position of the media only affect the current
   media being played. For example, once a media resource in the list
   completes, a 'reverse' operation on a subsequent media resource will not
   start a previously completed media resource at the appropiate offset.
 - This patch does not add any new operations to control the list. Hopefully,
   user feedback and/or future patches would add that if people want it.

ASTERISK-26022 #close

Change-Id: Ie1ea5356573447b8f51f2e7964915ea01792f16f

include/asterisk/stasis_app_playback.h

@@ -41,6 +41,8 @@ enum stasis_app_playback_state {
 	STASIS_PLAYBACK_STATE_PLAYING,
 	/*! The media is currently playing */
 	STASIS_PLAYBACK_STATE_PAUSED,
+	/*! The media is transitioning to the next in the list */
+	STASIS_PLAYBACK_STATE_CONTINUING,
 	/*! The media has stopped playing */
 	STASIS_PLAYBACK_STATE_COMPLETE,
 	/*! The playback was canceled. */
@@ -84,7 +86,8 @@ enum stasis_app_playback_target_type {
  * available codecs for the channel.
  *
  * \param control Control for \c res_stasis.
- * \param file Base filename for the file to play.
+ * \param media Array of const char * media files to play.
+ * \param media_count The number of media files in \c media.
  * \param language Selects the file based on language.
  * \param target_id ID of the target bridge or channel.
  * \param target_type What the target type is
@@ -95,8 +98,8 @@ enum stasis_app_playback_target_type {
  * \return \c NULL on error.
  */
 struct stasis_app_playback *stasis_app_control_play_uri(
-	struct stasis_app_control *control, const char *file,
-	const char *language, const char *target_id,
+	struct stasis_app_control *control, const char **media,
+	size_t media_count, const char *language, const char *target_id,
 	enum stasis_app_playback_target_type target_type,
 	int skipms, long offsetms, const char *id);
 
@@ -128,6 +131,14 @@ const char *stasis_app_playback_get_id(
  */
 struct stasis_app_playback *stasis_app_playback_find_by_id(const char *id);
 
+/*!
+ * \brief Convert a playback to its JSON representation
+ *
+ * \param playback The playback object to convert to JSON
+ *
+ * \retval \c NULL on error
+ * \retval A JSON object on success
+ */
 struct ast_json *stasis_app_playback_to_json(
 	const struct stasis_app_playback *playback);