Allow to publicly expose an @context from the forge

Currently, the project is protected, so we can't reference a JSON-LD @context hosted in the source code in the test cases because the SUT won't be able to resolve it.

We temporarily deployed a context on GitHub (https://raw.githubusercontent.com/easy-global-market/ngsild-api-data-models/feature/add-json-ld-context-for-ngsi-ld-test-suite/ngsi-ld-test-suite/ngsi-ld-test-suite-context.jsonld).

When the project is publicly opened, move this context somewhere in the source code.