Allow auto-reload of log4j2 config

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

spring-boot-samples/spring-boot-sample-actuator-log4j2/src/main/resources/log4j2.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<Configuration status="WARN">
+<Configuration status="WARN" monitorInterval="30">
     <Properties>
         <Property name="PID">????</Property>
         <Property name="LOG_PATTERN">[%d{yyyy-MM-dd HH:mm:ss.SSS}] log4j2%X{context} - ${sys:PID} %5p [%t] --- %c{1}: %m%n</Property>