Codegen feature: InputValueSerializer could autodetect extra scalar mappings based on registration in META-INF

[paulbakker]

For type mappings in Codegen we can register a mapping in META-INF for a library, so that Codegen automatically knows about the type. We can consider implementing a similar mechanism to register Coercing classes for scalars in InputValueSerializer, so that a user of a generated client API doesn't have to provide the mapping as follows explicitly:

GraphQLQueryRequest graphQLQueryRequest =
                new GraphQLQueryRequest(
                        query,
                        getEntityProjection(),
                        Map.of(URI.class, UriScalarImpl.INSTANCE.getCoercing()));