Support application-private messaging

[danielwiehl]

Is your feature request related to a problem? Please describe.

A microfrontend can communicate with its application activators, for example, using topic-based messaging. The problem is that those messages can also be received by other applications, or that other applications can respond to those requests, and not necessarily maliciously, for example in the case of name collisions.

Describe the solution you'd like

The platform should support application-private messaging, as follows:

• A message can be marked as application-private by setting a flag in the publishing options.
• Private messages are only routed between instances of the same application, thus other applications cannot receive private messages of other applications nor publish messages to private topics of other applications.
• Applications may define the same private topics. Still, private messages must never be routed between applications.