Avoid the static initializer in io.smallrye.graphql.schema.model.Scalars

[jmartisk]

While probably not a real problem for Quarkus applications, the static initializer brings a set of problems, because it acts upon the smallrye.graphql.federation-enabled property.

• It breaks WildFly with multiple deployments where some deployments have federation enabled and some don't, because the set of scalars is computed just once for the whole WildFly server.
• In tests, it forces us to create a separate JVM for each test because similarly to multiple deployments in WildFly, multiple tests in a single JVM are affected too.