Feature Request: Extend InternalLogger methods to take lamda parameters and only invoke these if the appropriate log level is enabled.

[JonPaz]

This feature idea has come from working with the NLog.Targets.Syslog target and an issue that I raised in that project (luigiberrettini/NLog.Targets.Syslog#56)

When making calls to the static InternalLogger, it is currently common to check the enabled log level before invoking the logger - particually when the information to be logged is the result of a potentially intensive operation such an ToString(), string concatination etc.

It would be useful, and provide safer / cleaner code, for the InternalLogger class to have additional methods that allow the client to provide a function that will only get invoked when the relevent log level is enabled, thus allowing optimal performance.

This has been initially discussed with @304NotModified as part of the issue mentioned above.