Retain the `IMqttRetainedMessagesManager` interface in version 4?

### Describe the feature request
It looks like the upcoming version 4 (referring to branch `feature/Reason_Code_In_Subscription_Interceptor`) no longer exposes the `IMqttRetainedMessagesManager` interface as an extension point. Using the `IMqttServerStorage` interface instead is fairly limiting and may not be the best option when dealing with physical storage (see also comments in #1206). So far the `IMqttRetainedMessagesManager` could be used to inject one's own retained message manager, which means one is not limited by `IMqttServerStorage` .

### Which project is your feature request related to?
- Server

### Describe the solution you'd like
It would be good to keep the `IMqttRetainedMessagesManager` extension point available in version 4.

### Describe alternatives you've considered
Alternatively the `IMqttServerStorage` could be modified to contain the methods that were previously in the `IMqttRetainedMessagesManager` interface.