Annotation of Consumer/Provider Type

When looking at a Celix Service Interface for the first time, it is pretty easy to get confused as to whether you should implement and register the service in the header file or just track and use the service. Though we don't support annotation directly in C++, adding Doxygen comments such as `@qualifer ProviderType` or `@qualifer ConsumerType` should be enough to help our users.

Note that [qualifer](https://www.doxygen.nl/manual/commands.html#cmdqualifier) is a standard Doxygen command.
If users hover over the type documented this way in an IDE like CLion, the qualifier (ProviderType/ConsumerType) will appear in the generated documentation.