check if registration already exists

[tstorek]

Is your feature request related to a problem? Please describe.
Currently, nothing progibts to post dublicate registrations to the context broker. This basically the same a with subscription #137
Hence, we require a strategy that checks for matching subscriptions or registrations before posting them and give at least a warning if it will dublicate an existing one.

Describe the solution you'd like

• 1. feat: compare subscriptions for different dict formats #138 is an excellent starting point. Let's go from there
• 2. implement a matching strategy for EntityPattern.
• 3. check other fields of subscriptions and registration if they match
• 4. Implement strategy if existing Subscription/Registration already covers the new one, e.g. raise an error.
• 5. Implement strategy if existing Subscription/Registration partially dublicates the new one, e.g. give a merge option.