[rdf] Improve support for JSON-LD Contexts (e.g. url prefixes/aliases)

[robvandenbogaard]

For interfacing with JSON-LD services using schema url prefixes in the Context to shorten urls in the content data representation we need to expand existing mod_ginger_rdf support as the expansion of prefixes is currently only implemented for predicates, not values/objects.

Background:
https://json-ld.org/spec/latest/json-ld-api-best-practices

Existing libraries:
Only one non-abandoned Elixir library seems to be available for the Erlang universe - https://github.com/marcelotto/jsonld-ex