Add Support for (annotated) GraphQL Schema

[Aklakan]

With the backbone for ad-hoc queries in place, it's time to add support for graphql schema. The main technical upgrade is that the source for field annotations may be based on schema. Right now, the only source for annotations are the directives present on the field itself.

Also, a question is how to supply the schema.

• One simple option is as a CLI argument.

rpt integrate --graphql:schema mapping.graphqls

• A REST API for updates (like Tentris) does not make much sense without protection. RPT could be extend with an --admin-password file.txt or even account.txt option. So far my perspective was that for more advanced features, one should use Fuseki with the JenaX plugin bundle JAR added to it - via our Fuseki Mods integration. The goal of RPT is to make it easy to work in an ad-hoc fashion with RDF, SPARQL and GraphQL; not to duplicate the work needed to create a proper RDF server framework with account management.