Add support for schema references

[kleewho]

This change adds the support for schema referencing. The references support has been added to apicurio registry I believe in version 2.2.2, but the sync operator was not able to handle that. The version of apicurio registry used in tests had to be bumped because version 2.1.5 was not supporting references and there has been quite a few fixes touching references in various releases therefore I landed on version 2.4.2.

After this the operator won't longer work with older versions of apicurio registry

[LittleWat]

Thank you for implementing this! We'd like to use this feature as well.

@carlesarnal sorry for mentioning you but could you review this PR, please...? 🙏

[carlesarnal]

@LittleWat I think I have mentioned this in past PRs, but this repository is no longer maintained and our approach for gitops will be quite different in 3.0. If you want to continue using it, it has to be community driven, so I can give you permissions to perform reviews if you're ok with that.

There is a number of reasons for not following this approach, some of them being that this is a "broken by design" approach as the current Apicurio Registry API is not built to be idempotent and used in such a way:

• reconciliation is going to fail under some (common) conditions like concurrent updates of artifacts
• detecting failures in reconciliation requires extra monitoring and external tools causing Registry to misbehave instead of failing
• manually fixing a failure in reconciliation always requires human intervention
• the amount of data that can be stored is limited (by default ETCD limit is, per object, 1.5MB)

I'll add a note to the README mentioning those reasons.