Filtering messages by a type of enclosed into `Any` field

[alexander-yevsyukov]

Currently we can subscribe either to event messages or to the whole Event type. It is not possible to subscribe to Event—so that both message and context can be observed—using the type of the enclosed event message. Our Topic API does not have such an ability.

What I want in the code to say is this:
— Send me all TaskCreated events (with contexts).
— Send me UserLoggedIn (with contexts) matching originating command ID. The latter is possible already.

Such a filtering feature may be also convenient not only for events, but also for entity states, where a field is Any and only certain states containing specific types are of interest.