logger: Add custom logging capabilities

Adds the ability for users to log to custom log levels
by providing custom log level names in logger.conf. Also
adds a logger show levels CLI command.

ASTERISK-29529

Change-Id: If082703cf81a436ae5a565c75225fa8c0554b702

include/asterisk/logger.h

@@ -329,6 +329,14 @@ unsigned int ast_debug_get_by_module(const char *module);
  */
 int ast_logger_register_level(const char *name);
 
+/*!
+ * \brief Retrieve dynamic logging level id
+ * \param name The name of the level
+ * \retval The unique integer id for the given level
+ * \retval -1 if level name not found
+ */
+int ast_logger_get_dynamic_level(const char *name);
+
 /*!
  * \brief Unregister a previously registered logger level
  * \param name The name of the level to be unregistered