Axum integration

[btielen]

This PR provides an integration for Axum including subscriptions (fixes #986)

• including http tests run by juniper::http::tests::run_http_test_suite
• including websocket tests run by juniper::http::tests::run_ws_test_suite

Get started

Run cargo run -p juniper_axum --example simple from the root directory, and navigate to 127.0.0.1:3000 in your browser.

Other changes

Minor changes were made in other libraries

• Added an Error for the implementation of Sink for juniper_graphql_ws::Connection, so the code doesn't panic.
• Added an extra test in juniper::http::tests::run_ws_test_suite to test POST requests with variables

Discussion

• Documentation probably needs some polishing
• juniper::http::GraphQLRequest now has a property variables with type Option<InputValue<S>>, I think (but not sure) things would be easier if this has the type Variables<S>
• I didn't test requests with an operation_name defined

Any help polishing this PR is appreciated :)

[btielen]

In case this PR will get merged, it would be nice to update the projects documentation and book to point out an integration for Axum exists.

[btielen]

Thanks for the feedback, I will take a look at it tomorrow.

[harksin]

:wave: Incredible work :star_struck: , Is there any plan to merge & release it ?