Subscription does not work using rpc calls

[eneufeld]

If you want to subscribe to changes from a modelserverclient which is exposed to the frontend using theias proxy, the subscriptions do not work.
The reason is probably that the registered callback is lost and must be offered differently.

[ndoschek]

To subscribe and receive all notifications on the frontend, it is recommended to use the concept of the ModelServerFrontendClient.
It is bound by default for v2 and can be accessed on the frontend by injecting the
ModelServerSubscriptionClient.

As an example, please see the usage of the subscription service in ApiTestMenuContribution.