Backwards compatible retention of extension parsing for async reqwest

[bengsparks]

Why are we making this change?

Partially addresses the needs of #927.

What effects does this change have?

Changes the ReqwestExt trait to return DelayedResponse.

Calling DelayedResponse::retain_extensions::<T> and awaiting will deserialise the extensions field using the shape of T.

Directly awaiting DelayedResponse (i.e. the previous behaviour, thereby maintaining backwards compatibility) will produce an ImmediateResponse, which will ignore the contents of the "extensions" field.

Adds futures-lite to make implementing of std::future::Future simpler.
If you think the implementation could be improved to avoid needing futures::FutureExt, please make any changes you deem necessary.

Adds tokio = { version = "1", features = ["macros"] } and mockito = "1.4.0" to dev dependencies to provide an asynchronous mock server for testing purposes.

cynic/tests/http.rs provides a comparison of their usages.

Examples:

For a GraphQL server that responds with

{
  "errors": [
    {
      "message": "Unauthorized",
      "locations": null,
      "path": [ "helloWorld" ],
      "extensions": { "code": 401 }
    }
  ]
}

Including extensions with DelayedResponse::retain_extensions

#[derive(Debug, serde::Deserialize)]
struct Extensions {
    code: u16,
}

let output = client
    .get(format!("http://{}/graphql", graphql.host_with_port()))
    .run_graphql(FieldWithString::build(FieldWithStringVariables {
        input: "InputGoesHere",
    }))
    .retain_extensions::<Extensions>()
    .await;
dbg(!&output);

Will produce a response with extensions: Some(Extensions { code: 401 } ).

Backwards compatible

let client = reqwest::Client::new();
let output = client
    .get(format!("http://{}/graphql", graphql.host_with_port()))
    .run_graphql(FieldWithString::build(FieldWithStringVariables {
        input: "InputGoesHere",
    }))
    .await;
dbg(!&output);

Will produce a response with extensions: Some(IgnoredAny).

[netlify]

✅ Deploy Preview for cynic-querygen-web canceled.

Name	Link
🔨 Latest commit	af3dcd8
🔍 Latest deploy log	https://app.netlify.com/sites/cynic-querygen-web/deploys/662e70a314e7df0008eb0bed

[bengsparks]

@obmarg The CI fails to due to some version incompatibility between clap and rustc. Please let me know if I need to make any changes to this PR myself to fix this.

[obmarg]

Nice work @bengsparks - thanks for taking the time to do this.

I'm going to push up a couple of tweaks:

1. rolling back some of the Cargo.toml changes (I do need to upgrade a lot of the packages, but I'd rather not bundle those updates into this PR)
2. I've removed the need for futures-lite - nothing wrong with it, but I'd rather avoid an extra dependency if I can.
3. Also renamed DelayedResponse to CynicReqwestBuilder. DelayedResponse makes sense in the context of this PR, but this builder seems like it'd be a good place to put any other features that get added to the integration, so I'd like to go for the more generic name.

[obmarg]

Released in v3.7.0