Register directives via schema config

[sforward]

Summary

With this PR you could register directives. This opens the option to explore the possibilities of directives.

Do you want me to add this to the readme? Subscriptions can be registered the same way, but aren't mentioned in de readme.

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Type of change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Misc. change (internal, infrastructure, maintenance, etc.)

Checklist:

• Existing tests have been adapted and/or new tests have been added
• Add a CHANGELOG.md entry
• Update the README.md
• Code style has been fixed via composer fix-style

[mfn]

We had multiple attempts in the past ( #703 , #860 ).

What is the end goal here? I think I'd be more confident knowing where this goes, as there were reasons why the previous approaches didn't pan out.

[sforward]

My previous attempt was more focussed on a general support for directives. The current PR makes it just possible to register directives to the schema, so anyone can add specific functionality via a custom directive.

For example, I was attempting an execution middleware that checks for a directive on the query level to apply some specific caching. But also directives on the field level like added to the tests in #860 could be implemented with this PR, if someone needs it in an application.

[mfn]

Would it be possible to provide a small / non-useless directive as an example, which we then further could also use in a test?

Do you want me to add this to the readme?

If we can make the above work, I'd be for this!

[sforward]

I've added a test for the directive registration. It includes an example directive with all available options.

[mfn]

LGTM, thank you!