Support the HTTP QUERY method (RFC 10008)

Support for the HTTP QUERY method (RFC 10008) is being worked on upstream, both in the GraphQL spec and in Go:

https://github.com/graphql/graphql-over-http/pull/411 — adds QUERY to the GraphQL-over-HTTP specification. https://github.com/golang/go/issues/80058 — adds http.MethodQuery to net/http.

gqlgen currently has transport.GET and transport.POST but no transport for QUERY, so QUERY requests are rejected with transport not supported.

I think it’s fine to revisit this in gqlgen once support for the GraphQL specification and Go query methods has been released.