allow directives on directive definitions

[benhead]

This is the minimum viable edit to enable this feature. I imagine we'd also want to allow directive extensions if this feature is approved.

#566

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[benhead]

Original feature request:

We'd like to be able to declare directives on directive definitions.

We have tooling that uses type system directives to perform special processing on a schema, and we'd like users of that tooling to be able to define their own directives, with metadata indicating to the tooling that this directive is for its use. For example, if we could define directive @Handler(class:String!) on DIRECTIVE_DEFINITION then our users could define directive @CustomDirective(...) on FIELD_DEFINITION @Handler(class:"Foo") telling our tooling that their directive is meant to be handled by it, passing arguments we declare to our tooling, and still providing whatever arguments they declare to their own hooks.

That directives can't be declared on directive definitions seems like an oversight.

Please note that I'm a Googler and we're reviewing that we can sign the CLA. I don't anticipate a problem, but bear it in mind until we confirm.

[spawnia]

I would like to another use case that requires this change that might be a bit more common: Deprecation of directives.

directive @foo on OBJECT @deprecated