Support multiple operations per query document

[tomhoule]

Probably via a struct attribute on the query struct to specify which operation it should implement.

For now we should document that we only support one operation by document (like Apollo Client, I think?)

[tomhoule]

This is important because it's the most natural way to share fragments between multiple operations (an operation is either a query, a mutation or a subscription).

[tomhoule]

I started implementing that - the question is whether we should keep the 1:1 match between a struct under derive and an operation, or generate multiple implementors of the GraphQLQuery trait (one for each operation).

I tend to prefer the first option, as it's more explicit and gives more control to users (and it's not a lot of boilerplate to define additional structs with nearly the same options but a different name).

[tomhoule]

Question: should we select the operation with the name of the struct by default, or the first one, or a combination of both? I think the struct name, with a fallback on the first one makes sense - or maybe struct name, and error if there is no match.