[1.x] Enable IntegrationBroker dispatch events regardless of registration order of subscribing and publishing BCs

[yevhenii-nadtochii]

Now, IntegrationBroker sends out an ExternalMessagesSourceAvailable notification as it registered with a BoundedContext in order to notify other brokers that a new potential contributor is available. Other brokers react on this notification by re-stating their needs of external events by sending a RequestForExternalMessages.

Notes

1. IntegrationBroker tests were re-written with BlackBoxContext;
2. One test seemed not to be anyhow related with IntegrationBroker. Instead of checking for UnsupportedExternalMessage as it stated in the title, it was checking for UNSUPPORTED_EVENT_VALUE that is a responsibility of EventBus.

    @DisplayName("emit unsupported external message exception if message type is unknown")
    void throwOnUnknownMessage() throws Exception {
        ....
        assertEquals(UNSUPPORTED_EVENT_VALUE, error.getCode());
    }

Non-related changes

1. Made BoundedContext.InternalAccess.register*() methods chainable;
2. Fixing caught typos in javadoc;
3. EmptyAggregate in server/src/test/proto/spine/test/shared_types.proto had incorrect entity type;
4. Made BlackBoxContext implement Closeable.

[codecov]

Codecov Report

Merging #1402 (8bf4de1) into 1.x-dev (2cfad47) will decrease coverage by 0.00%.
The diff coverage is 95.65%.

@@              Coverage Diff              @@
##             1.x-dev    #1402      +/-   ##
=============================================
- Coverage      91.05%   91.05%   -0.01%     
- Complexity      4789     4791       +2     
=============================================
  Files            614      614              
  Lines          15185    15201      +16     
  Branches         861      860       -1     
=============================================
+ Hits           13827    13841      +14     
- Misses          1085     1091       +6     
+ Partials         273      269       -4     

[armiol]

@yevhenii-nadtochii please see my comments.

Also, next time please mark the PR as "Ready for review". It's in its "Draft" state at the moment.

[armiol]

@yevhenii-nadtochii one more thing. We'll need to bump the library version. I'll show you how to do that.