cel: Add STREAM_BEGIN, STREAM_END and DTMF event types.

Fixes: #1280

UserNote: Enabling the tracking of the
STREAM_BEGIN and the STREAM_END event
types in cel.conf will log media files and
music on hold played to each channel.
The STREAM_BEGIN event's extra field will
contain a JSON with the file details (path,
format and language), or the class name, in
case of music on hold is played. The DTMF
event's extra field will contain a JSON with
the digit and the duration in milliseconds.

(cherry picked from commit 0906282f25)

include/asterisk/cel.h

@@ -77,6 +77,12 @@ enum ast_cel_event_type {
 	AST_CEL_LOCAL_OPTIMIZE = 17,
 	/*! \brief A local channel optimization has begun */
 	AST_CEL_LOCAL_OPTIMIZE_BEGIN = 18,
+	/*! \brief A stream started */
+	AST_CEL_STREAM_BEGIN = 19,
+	/*! \brief A stream ended */
+	AST_CEL_STREAM_END = 20,
+	/*! \brief A DTMF digit was processed */
+	AST_CEL_DTMF = 21,
 };
 
 /*!