Support for logging in JSON and addition of extra information fields in the info actuator

[tbeekman]

This PR adds two additional functionalities which are used in our environment:

• Logging in JSON format
• Additional information fields in the info actuator

Logging in JSON format
If enabled, the logback-spring.xml configuration can be used to create logfiles in JSON format next to the default line-format. JSON-formatted log files can easily be fed to Elastic stack and Graylog environments using Filebeat.

Additional information fields in the info actuator
Additions have been added to expose the following fields in the info actuator under "build":

• version (application version)
• artifact (name of the artifact)
• java_version (used version of Java by the application)
• name (name of the application)
• time (build time)
• spring_boot_version (version of Spring Boot used by the application)
• group (artifact group name)

Additional field added in the info actuator:

• days_since_release (number of days passed since the release was build; eases possibilities to monitor how old the release is and to determine if a new build should be made)

Monitoring software can be pointed to these fields to have a better grasp on applications currently running and improve the application lifecycle management.