Allow JSON-LD context as referenced URL

Currently, when submitting VPs, the catalog allows embedded context like:

"@context": { "gx": "https://registry.lab.gaia-x.eu/development/api/trusted-shape-registry/v1/shapes/jsonld/trustframework#" }

But fails with referenced context which lists URLs, like in the Gaia-X Lab credentials:

"@context": [ "https://www.w3.org/2018/credentials/v1", "https://w3id.org/security/suites/jws-2020/v1", "https://registry.lab.gaia-x.eu/development/api/trusted-shape-registry/v1/shapes/jsonld/trustframework#" ]

JSON-LD (1.0 and 1.1) allows both. This was found out and is related to #199 (closed).