Write ArchUnit rule to prevent Stream from being used as a field in a class

[tedyoung]

I call this rule "Field of Streams". For now, finding a field in a class (or Record!) that is of type Stream (any generic type) should be a test failure. This might need to be relaxed in the future, but for now it's a failure.

This is unrelated to Hexagonal Architecture, so should be in a new test class in the main package, i.e., package com.jitterted.mobreg; and be tagged with @Tag("architecture"), because ArchUnit tests are a bit slow, so only want to run on GitHub.