Implement Astarte Identities

[bettio]

Astarte Identities purpose is to enable decoupling of data from the devices that produce data.
They should cover following scenarios:

• Mapping physical sensors (having their unique id) that might connect at any moment to different gateways which they publish data on their behalf
• Maintaining a machine history when the physical device that publishes data is replaced

This should be done by allowing mapping some parametric endpoints or by assigning the same identities to different physical device at different times.