Logger callback function sink (#17095)

Migrated from #1748

This PR implements
#1330 through a new
logger sink: a user configurable callback. It introduces some spec
additions:

- `typedef void (*ur_logger_output_callback_t)(ur_logger_level_t level,
const char *pLoggerMsg, void *pUserData)`
- `urSetLoggerCallback(ur_adapter_handle_t hAdapter,
ur_logger_output_callback_t pfnLoggerCallback, void *pUserData,
ur_logger_level_t level`
)`
- `urSetLoggerCallbackLevel(ur_adapter_handle_t hAdapter,
ur_logger_level_t level)`
- `typedef enum ur_logger_level_t` (moved the logger::level enum into
the spec)

This new logger sink will only be constructed once a user makes a call
to `urSetLoggerCallback` (which is called from the UR
`urAdapterSetLoggerCallback` entry point), supplying their own callback
function. They can set the minimum logging level through
`urSetLoggerCallbackLevel`. Any subsequent logging calls will
additionally make a call to the supplied callback where the log level,
message and user data will be sent.

A callback has been setup in the SYCL RT in `sycl/source/detail/ur.cpp`
to print logs to `std::err`:
```
void urLoggerCallback([[maybe_unused]] ur_logger_level_t level, const char *msg,
                      [[maybe_unused]] void *userData) {
  std::cerr << msg << std::endl;
}
```

A new test suite `LoggerWithCallbackSink` has been added to test this
new functionality.

---------

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

Author: martygrant

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: