(feat) - allow passing operationContext to useQuery

[JoviDeCroock]

Fixes: #346

[kitten]

If there are no objections, I'd just like to merge this as is for now 😅 Potentially this could obviously be nicer, but since for most users context won't be passed, the simple solution may be best here. In other cases for "advanced" use-cases with context, I'd assume that people would spot the required memoisation rather quickly.

On this note though, we may want to add the same modification to useMutation and useSubscription as well:

• useMutation: Potentially an optional second argument on executeMutation? https://github.com/FormidableLabs/urql/blob/master/src/hooks/useMutation.ts#L35
• useSubscription: Possibly the exact same https://github.com/FormidableLabs/urql/blob/master/src/hooks/useSubscription.ts#L12

Edit: Actually let's merge this for now so that we don't increase the scope of this PR :)

[JoviDeCroock]

Hmm, weird issue in the useSubscription test. When logging it calls with url but when checking the mock it doesn't seem to be present. Will look further after a break

[JoviDeCroock]

@kitten just updating docs atm and then I think it's ready to merge, is there a spot that should be safe to note about the context and memoization of it?

[kitten]

@JoviDeCroock I suppose we should add an entire section about the context somewhere, where we're also talking about customising urql. So maybe in "Extending & Experimenting". I'm thinking that doc can be extended with "FAQ-style" guides.

E.g.: "How to write custom exchanges", "How to extend the context", etc

[JoviDeCroock]

All right, added it thanks for the heads up. It's rather small now but we can iterate on this.

[kitten]

@JoviDeCroock let's discuss this tomorrow! There's a lot of todos for the docs anyway, so we can put down some specific todos for what sections need to be written 🙌