Provide an Abstractions package

### Describe the feature request
I want to be able to have a library which handles mqtt messages without needing to take on a dependency of the entire mqtt library which includes a client etc.

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

- Client
- ManagedClient
- Server
- Generic

### Describe the solution you'd like
I want to be able to install a nuget package containing just the classes needed to handle a message. It might be worthwhile to also include a couple of other interfaces ie IMQTTClient

### Describe alternatives you've considered
Accepting the way it is.

### Additional context
Want to also improve the cloud events SDK