Support waiting command gateway.

[zambrovski]

A waiting command gateway should wait with the resolution of completable future until the projection update is delivered.

[zambrovski]

Here is a workaround for this by Steven.
https://stackoverflow.com/questions/64045477/how-to-make-command-to-wait-until-all-events-triggered-against-it-are-completed/64117171#64117171

Here is again a question on it in the forum:
https://discuss.axoniq.io/t/how-to-make-command-to-wait-until-all-events-triggered-against-it-are-completed-successfully/2724

[lfgcampos]

This might also be a source for the 'waiting command gateway'
https://github.com/AxonIQ/code-samples/tree/master/subscription-query-rest