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 ?

[urothis]

Any blockers on merging this?

[Cugatay]

Hey, are you going to push Axum integration? I loved Axum when I tried it, and I want to use something I already know when using GraphQL

[LegNeato]

There are still outstanding review comments here...

[frederikhors]

@btielen are you able to fix those changes so to merge this?

[tyranron]

I plan to finish it in next few weeks.

[tyranron]

Progress

1. Reverted graphql_ws::WebsocketError changes, as they're clearly behind theme of this PR.
2. The same with Cloneing of RootNode. Such refactoring is better to be done properly in a separate PR (if makes sense), as touches all the integration crates.
3. Added CI integration for juniper_axum.
4. Added GraphiQL endpoint to juniper_axum.
5. Migrated to 0.6 axum (without subscriptions yet).
6. Got rid of GetQueryVariables and JsonRequestBody, which duplicated GraphQLRequest.

[tyranron]

Progress

1. Parametrized extractor::JuniperRequest with ScalarValue.
2. Reworked legacy graphql-ws GraphQL over WebSocket Protocol implementation.
3. Fixed doc tests.
4. Added new graphql-transport-ws GraphQL over WebSocket Protocol implementation.
5. Implemented auto-selection between new graphql-transport-ws and legacy graphql-ws GraphQL over WebSocket Protocol implementations.
6. Add ready-to-go default graphql, subscriptions::ws, subscriptions::graphql_ws and subscriptions::graphql_trasnport_ws handlers.
7. Polished API docs.

[tyranron]

@LegNeato I'm going to merge this in next few days. Take a final look, if you would like. All the stuff is ready, only minor CI issues are left to be resolved.