Add a CallerContext getter on the Schedulable interface

We would like to add a default CallerContext getter on the Schedulable interface

default public CallerContext getCallerContext() {
  return null;  
} 

and then override it on the ipc/Server.Call class

@Override
public CallerContext getCallerContext() {  
  return this.callerContext;
} 

to expose the already existing callerContext field.

This change will help us access the CallerContext on an Apache Ozone IdentityProvider implementation.

On Ozone side the FairCallQueue doesn't work with the Ozone S3G, because all users are masked under a special S3G user and there is no impersonation. Therefore, the FCQ reads only 1 user and becomes ineffective. We can use the CallerContext field to store the current user and access it on the Ozone IdentityProvider.

This is a presentation with the proposed approach.

https://docs.google.com/presentation/d/1iChpCz_qf-LXiPyvotpOGiZ31yEUyxAdU4RhWMKo0c0/edit#slide=id.p