GraphQL G0: module and spike of server and client adapters #1582

Part of #1581.

Create org.restlet.ext.graphql with a first GraphqlServerAdapter and GraphqlClientAdapter.

Acceptance:

• A query and a mutation round trip on one HTTP/2 port next to a REST resource.
• Named-module check: graphql-java 26.1, java-dataloader and reactive-streams have no module descriptor; their automatic module names (com.graphqljava, org.dataloader, org.reactivestreams) must resolve on the module path from a named consumer.
• Data fetchers run on the call's virtual thread; CompletableFuture fetchers are joined there.
• Module depends only on org.restlet, graphql-java and jackson-databind 3.