[UR] Added new output sink to the logger which is a user configurable callback function. Configurable through two new entry points with adapter implementations: urAdapterSetLoggerCallback() and urAdapterSetLoggerCallbackLevel(). Moved logger::level enum to the spec, named ur_logger_level_t. Added new unit test suite for these entry points.

Co-authored-by: Kenneth Benzie (Benie) <[email protected]>

Author: martygrant

unified-runtime/include/ur_api.h

@@ -202,9 +202,9 @@ By default, no messages are printed.
 
 By default, there is a guarantee that *error* messages are flushed immediately. One can change this behavior to flush on lower-level messages.
 
-Loggers redirect messages to *stdout*, *stderr*, or a file (default: *stderr*).
+Loggers redirect messages to *stdout*, *stderr*, a file, or a user configurable callback function (default: *stderr*).
 
-All of these logging options can be set with **UR_LOG_LOADER** and **UR_LOG_NULL** environment variables described in the **Environment Variables** section below.
+All of these logging options (except the callback) can be set with **UR_LOG_LOADER** and **UR_LOG_NULL** environment variables described in the **Environment Variables** section below.
 Both of these environment variables have the same syntax for setting logger options:
 
   "[level:debug|info|warning|error];[flush:<debug|info|warning|error>];[output:stdout|stderr|file,<path>]"
@@ -232,6 +232,10 @@ An example of an environment variable for setting up the null adapter library wi
 
   UR_LOG_NULL="level:warning;output:stdout"
 
+Logging callback
+^^^^^^^^^^^^^^^^^^^^^
+An API is available to configure the logging callback function :ref:`urAdapterSetLoggerCallback`. Additionally, the logging level can be set using :ref:`urAdapterSetLoggerCallbackLevel`.
+
 Adapter Discovery
 ---------------------
 UR is capable of discovering adapter libraries in the following ways in the listed order: