Add opt-in support for registering a shutdown hook to shut down logging

This commit adds a new property, logging.register-shutdown-hook, that
when set to true, will cause LoggingApplicationListener to register
a shutdown hook the first time it initializes a logging system. When
the JVM exits, the shutdown hook shuts down each of the supported
logging systems, ensuring that all of their appenders have been
flushed and closed.

Closes gh-4026

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -65,6 +65,7 @@ content into your application; rather pick only the properties that you need.
 	logging.pattern.console= # appender pattern for output to the console (only supported with the default logback setup)
 	logging.pattern.file= # appender pattern for output to the file (only supported with the default logback setup)
 	logging.pattern.level= # appender pattern for the log level (default %5p, only supported with the default logback setup)
+	logging.register-shutdown-hook=false # register a shutdown hook for the logging system when it is initialized
 
 	# IDENTITY ({sc-spring-boot}/context/ContextIdApplicationContextInitializer.{sc-ext}[ContextIdApplicationContextInitializer])
 	spring.application.name=