Add API for channel frame deferral.

There are several places in Asterisk that have duplicated logic
for deferring important frames until later.

This commit adds a couple of API calls to facilitate this automatically.

ast_channel_start_defer_frames(): Future reads of deferrable frames on
this channel will be deferred until later.

ast_channel_stop_defer_frames(): Any frames that have been deferred get
requeued onto the channel.

ASTERISK-26343

Change-Id: I3e1b87bc6796f222442fa6f7d1b6a4706fb33641

main/channel_internal_api.c

@@ -221,6 +221,7 @@ struct ast_channel {
 	struct stasis_cp_single *topics;		/*!< Topic for all channel's events */
 	struct stasis_forward *endpoint_forward;	/*!< Subscription for event forwarding to endpoint's topic */
 	struct stasis_forward *endpoint_cache_forward; /*!< Subscription for cache updates to endpoint's topic */
+	struct ast_readq_list deferred_readq;
 };
 
 /*! \brief The monotonically increasing integer counter for channel uniqueids */
@@ -1681,3 +1682,8 @@ enum ast_channel_error ast_channel_internal_errno(void)
 
 	return *error_code;
 }
+
+struct ast_readq_list *ast_channel_deferred_readq(struct ast_channel *chan)
+{
+	return &chan->deferred_readq;
+}