Endpoint for GTT context entities

[dkastl]

To promote interoperability of data exchange, NGSI-LD context brokers explicitly expose a JSON-LD @context file to define the data held within the context entities. This defines a unique URI for every entity type and every attribute so that other services outside of the NGSI domain are able to pick and choose the names of their data structures.

Every @context file must be available on the network. In GTT case these files can be served from a GTT server instance and be part of the plugin.