Support derives on enum declarations

[LucasPickering]

Hey, got another feature request for you. There's a lot of times where I'd like to derive some trait for an enum generated by graphql_schema_from_file. Currently I'm struggling with an enum generated for a union type that I need Clone on. Of course the workaround is to just implement Clone manually, but it definitely would be nice to have a derive. It would also be especially nice to be able to apply other attributes beyond derive to the struct definitions, e.g. serde attributes to customize serialization/deserialization.

I'm not sure what the implementation for this would look like, but I assume on the schema side we would have to define these via a directive. According to the graphql schema spec, directive values can be any type, so I'm thinking maybe a new directive field with a list of strings? Something like this:

type A {}

type B {}

@juniper(attributes: ["derive(Clone, Debug, Serialize)", "serde(tag = "type")"])
union U = A | B

What do you think? If this seems viable and you give me some direction on how to implement it, I can work on the actual code for it.