Extensibility refactoring
 - RegistryInjector renamed to SerializerRegistryInjector
 - EnhancedMessageTypeKey moved to api project to be visible outside library implementation
 - SerializerRegistryImpl - unified Exception message
 - MessageTypeKey - updated hashCode()
                  - also updated SerializationFactory key to .getImplementedInterface() instead of .getClass()

Signed-off-by: Michal Polkorab <michal.polkorab@pantheon.sk>