Log4j2 can auto-reload its configuration file as long as the reference
to a `java.io.File` is provided in the `ConfigurationSource`. Previously,
we always created such `ConfigurationSource` with only the URL regardless
of its type.

Detect when the configuration URL points to a File and create the
`ConfigurationSource` accordingly.

The `spring-boot-sample-actuator-log4j2` has been updated to reload the
logging configuration every 30 sec if necessary.

Fixes gh-3024, gh-3030