please add functional directives

[bionicles]

Is your feature request related to a problem? Please describe.
I would like to use Juniper but unfortunately, it's critical for my use case to be able to assign functions to custom directives for fine-grained attribute-based access control

Describe the solution you'd like
some way to define a directive and assign that to a function. basically, the same thing as graphql-tools.js directiveResolvers, where you pass a directive-name: function pair, and it just makes a directive for each function. Multiple directives can stack, like middleware, using some next() function

Describe alternatives you've considered
I guess I could use Node.js and graphql-tools but I really want to play with rust because I admire the language. Just can't sacrifice security of these directives

Additional context
an auth directive could perform some function on the user, database tuple, and return a boolean authorized/unauthorized; users could use this to quickly and cleanly secure their api

[mathroc]

not the same, but you can do that inside your resolvers : https://graphql-rust.github.io/types/objects/using_contexts.html

and return an Option<T> or FieldResult<T>