Support custom Log4J2 log levels by changing `LoggerConfiguration` so
that it can now report levels using a `LevelConfiguration` object
rather than the limited `LogLevel` enum.
The `Log4J2LoggingSystem` class now uses `LevelConfiguration.ofCustom`
for custom logging levels, rather than throwing an exception.
The `LoggersEndpoint` has also been updated so that it can return the
custom logger name.
Fixes gh-35227
137f4ee451