Introduce contract tests

[holly-cummins]

The registry is a great use case for contract tests, since it depends on behaviour in the main Quarkus build, and since http://code.quarkus.io, the Quarkus CLI, and http://quarkus.io/extensions depend on registry behaviour.

Some issues related to interaction between these systems, such as quarkusio/quarkus#31569, might have been avoided with contract tests. Cross-system integration tests would also catch these issues, but are much heavier.

Based on what people report accidentally breaking :), I believe we need contract tests in both directions, quarkus build -> registry, and registry -> cli/code.quarkus/extensions.quarkus.