Add composeDirective directive, fixes #72

[cschiewek]

Pretty straight forward addition of the composeDirective directive.

I also considered adding a compose macro to notation to facilitate expansion, so that the link and composeDirective would be automatically added to the schema without having to add them to the extend schema block:

directive :custom do
  compose "https://spec.example.org/custom/v1.0"
  on [:schema]
end

I decided against it for this PR, but if that is something y'all think is worth the effort, let me know and I'll create an issue and do it at some point.

Cheers!

[kzlsakal]

@cschiewek thank you for the PR!

I also considered adding a compose macro to notation to facilitate expansion, so that the link and composeDirective would be automatically added to the schema without having to add them to the extend schema block

This is a very nice idea, although it's fine to support @composeDirective without that convenience for now.