[Rust] Add support for reqwest-middleware when using reqwest

Created by: whirm

Add support for reqwest-middleware (only when async support is enabled).

With this, one can easily add middleware which add features like tracing or retries to the client requests:

use petstore_async::apis::configuration::Configuration;
use reqwest::Identity;
use reqwest_middleware::ClientBuilder;
use reqwest_tracing::TracingMiddleware;

let config = Configuration {
        client: ClientBuilder::new(reqwest::ClientBuilder::new().build()?)
            .with(TracingMiddleware::default())
            .build(),
        ..Configuration::default()      
    }

Not sure if this would be considered a breaking change or not as it changes the Configuration struct signature. Let me know if I need to retarget the PR.

@frol, @farcaller, @richardwhiuk, @paladinzh, @jacob-pro: Pinging you as requested on the PR template :)

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