Add support for custom directives

[jangko]

we have at least two types of directives:

1. schema directives
2. query directives

but we also have lots of unanswered questions:

1. where in the execution/validation pipeline we should put custom directives execution?
2. how do we expose the internal of nim-graphql to custom directives?
3. how custom directives can safely manipulate things/AST?