added gx schema to defaults
the schema declares gx
prefix: @prefix gx: <https://registry.lab.gaia-x.eu/development/api/trusted-shape-registry/v1/shapes/jsonld/trustframework#> .
it allows SDs referencing https://registry.lab.gaia-x.eu/development/api/trusted-shape-registry/v1/shapes/jsonld/trustframework# context to be validated against Schema properly.
Addresses first point in #199 (closed). @jxhn, @fabianscheidt, @schuberr any feedback?