Add final initialization to ClientBuilder

[fivlao]

Add feature ClientBuilder::with_final_init and ClientBuilder::with_arc_final_init

This allows you to access extensions that are added using RequestBuilder::with_extension when creating a request.

Example:

use reqwest_middleware::{ClientBuilder, RequestBuilder, RequestInitialiser};

#[derive(Clone)]
struct Mark;

struct Init;

impl RequestInitialiser for Init {
    fn init(&self, mut req: RequestBuilder) -> RequestBuilder {
        let exists = req.extensions().get::<Mark>().is_some();
        assert!(!exists);
        req
    }
}

struct FinalInit;

impl RequestInitialiser for FinalInit {
    fn init(&self, mut req: RequestBuilder) -> RequestBuilder {
        let exists = req.extensions().get::<Mark>().is_some();
        assert!(exists);
        req
    }
}

#[tokio::main]
async fn main() {
    let request_client = reqwest::ClientBuilder::new()
        .build()
        .unwrap();
    let client = ClientBuilder::new(request_client)
        .with_init(Init)
        .with_final_init(FinalInit)
        .build();
    let resp = client.get("https://truelayer.com")
        .with_extension(Mark)
        .send().await
        .unwrap();
    println!("TrueLayer page HTML: {}", resp.text().await.unwrap());
}