@graphql-eslint/known-directives - allow specifying extra whitelisted directives

[osdiab]

Is your feature request related to a problem? Please describe.

I am using https://github.com/0no-co/gql.tada which introduces an @_unmask directive which isn't in my actual schema. I can't use it without disabling this ESLint rule, which is a bummer

Describe the solution you'd like

allow specifying that @_unmask is allowed in addition to whatever is in my schema

Describe alternatives you've considered

just disabling it when I use it

Additional context