Add support for SONAR_PATH_EXTENSIONS

I’d like to suggest a configuration parameter for the sonar.properties file and the associated environment variables. There are currently properties for sonar.path.data and sonar.path.temp which are helpful when attempting to redirect persistent data outside the application folder. I’d like to suggest one be added for sonar.path.extensions so that admins can tell sonar to look for extensions in a location other than the default application folder using a runtime variable.

In particular my intent would be to set this value in the environment variables as part of a docker compose file.