Full link to SensorML in /extra

[nuest]

Given the new flexible extra endpoint, for some applications it could be useful to offer more detailed and standardized information about stations/sensors, by providing a full URL to the original SensorML document. I suppose this works best with the SOS proxy. With direct DB access, this component could generate a full URL itself based on a configurable base URL.

In addition, different encodings of the full SensorML could be offered, since the 52N SOS supports JSON as well. Just list both SensorML/XML and JSON.

[SpeckiJ]

Progress can be tracked in PR #385