Handle minor version updates for server owned interfaces

[matt-mazzucato]

According to the versioning scheme for interfaces, new endpoints can be added to an interface with a simple minor version bump. Compatibility with former versions should be ensured.

Consider the following scenario: a server owned interface is updated by bumping its minor version and adding a number of new endpoints.
When a user sends data over that interface, Astarte only performs checks on the interface name and major version which are declared in the device introspection, and let the data flow towards the device. However, the device might have an old interface (with a different minor version) and, in that case, it might receive data on unknown endpoints.

It is not clear how this situation should be handled. There are some possible approaches to be discussed:

• thoroughly document the behavior and highlight the expected behavior when updating server owned interfaces;
• devices should drop data on "unknown" endpoints (this would lead to a silent drop of data without notifying the sender);
• Astarte should perform an additional check on the minor version of interfaces (this might be tricky as only the last minor version of an interface is tracked - previous versions are considered compatible by design).