Cleanup nifi-mock dependencies

We have allowed quite a few dependencies to creep into the nifi-mock module. It has dependencies now on nifi-utils, nifi-framework-api, nifi-parameter. These are not modules that the mock framework should depend on. We should ensure that we keep this module lean and clean.

I suspect removing these dependencies from the mock framework will have a trickle-down effect, as most modules depend on this module, and removing these dependencies will likely require updates to modules who use these things as transitive dependencies.

It appears that nifi-parameter is not even used, even though it's a dependency. There are two classes in nifi-utils that are in use: CoreAttributes and StandardValidators. But I argue these really should move to nifi-api, as they are APIs that are widely used and we will guarantee backward compatibility.

Additionally, StandardValidators depends on FormatUtils. While we don't want to bring FormatUtils into nifi-api, we should introduce a new TimeFormat class in nifi-api that is responsible for parsing things like durations that our extensions use ("5 mins", etc.) This makes it simpler to build "framework-level extensions" and allows for a cleaner implementation of NiFiProperties in the future. FormatUtils should then make use of this class.