Partner can subscribe for LO changes

As a partner, I'd like to subsribe for changes to a particular type of logistics objects.

Acceptance criteria

• A subscription request can be issued as a response to a previous subscription proposal, or triggered independently.
• The subscription API allows subscription requests for a particular type of logistics objects.
• The subscriber responds as defined by the specification (including a callback URL to receive published events).
• The server registers the provided callback URL and associates it for the given type of logistics objects.