[gql_http_link] Add response headers to context

[micimize]

In graphql-flutter I closed an issue about a specific solution for accessing response headers a while ago, but never followed up with the correct solution (adding headers to context)

zino-hofmann/graphql-flutter#371

[joshuadeguzman]

Hey @micimize,

I am using cookies for authenticating my requests. I found a workaround) at least for now for achieving it.

Curious about your thoughts in terms of performance passing a custom http client.

Thank you!

[micimize]

@joshuadeguzman any performance hit would be due to the overall performance diff between dio and http, which I would expect to be at most negligible. Also http also has some serious long-standing open issues that do not inspire confidence

[jeffscaturro-aka]

Cross-posting the tl;dr of zino-hofmann/graphql-flutter#704 (comment) here for more visibility and a reminder: we stuff the raw response object into the context that we can extract from a custom link.