Add extensions from ProtoData

[alexander-yevsyukov]

This PR brings the following improvements that were previously introduced in ProtoData:

• Just<E> tuple for one EventMessage. It's convenient to return in the places that need to return Iterable. Just<MyEvent> gives information about having only one event, which Iterable<MyEvent> cannot say.

• Policy<E> — is a producer of event(s) in response to incoming event. Provides nice method whenever().

• Querying<P> and QueryingClient<P> provide convenience methods for obtaining projection states. See QueryingClientSpec for details.

Other notable changes:

• SingleCommandAssignee was introduced (as the result of writing test fixtures context for QueryingClientSpec).
• BoundedContextBuilder got method addAssignee() which does the same as addCommandDispatcher() but is less mind-bending for people who did not participate in core-java development.
• Dependency version extra variables were removed.

[codecov[bot]]

Codecov Report

Merging #1489 (1d5bf9a) into master (164ebf1) will decrease coverage by 0.40%. The diff coverage is 69.11%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1489 +/- ## ============================================ - Coverage 90.07% 89.66% -0.41% - Complexity 4891 4896 +5 ============================================ Files 627 631 +4 Lines 15237 15302 +65 Branches 875 880 +5 ============================================ - Hits 13725 13721 -4 - Misses 1207 1275 +68 - Partials 305 306 +1 ```