feat(GraphQL): support application/graphql-response+json requests

[phryneas]

This would add support for the GraphQL over HTTP spec: https://github.com/graphql/graphql-over-http/blob/main/spec/GraphQLOverHTTP.md#accept

The spec introduces a new mime type that future severs should be able to handle - 'application/graphql-response+json' instead of the traditional 'application/json'.

So right now, all tests using HttpResponse.json would need to switch over to a new constructing function.
My suggestion here would be to just have GraphQLHandler return based on a list of supportedMimeTypes and the priority specified by the GraphQL client.
Clients requesting 'application/json' would be served 'application/json' while clients requesting "application/graphql-response+json, application/json;q=0.9" would be served 'application/graphql-response+json'.

Note that it would be possible to still always return 'application/json' and simulate a server that only speaks the old protocol by specifying a supportedMimeTypes of ['application/json'].

This is still lacking tests, but there is a good chance you want completely different behaviour so I'm opening this as-is for discussion.

[phryneas]

I'm sure there's a more elegant way to do this that I'm just not aware of....

[kettanaito]

Hi, @phryneas. Thanks for opening this!

It would help a lot if we started from the intended usage experience. How do you see people using MSW to mock queries done via the GraphQL-over-HTTP spec? Let's design the ideal usage and then work down from there to see what implementation that would require.

From my limited understanding, all that should happen is that mocked responses from graphql.* handlers should now contain the content-type: application/graphql-response+json header. Does anything else change for the developer who wishes to mock that standard?