Add option to disable http2 upgrade

[ducaale]

This can be helpful when there is a need to make an HTTP/1.0 or HTTP/1.1 connection but the server supports upgrading to HTTP/2.

let client = reqwest::Client::builder()
    .use_rustls_tls()
    .http1_only()
    .build()?;
let request = client
    .request(reqwest::Method::GET, "https://www.google.com")
    .version(reqwest::Version::HTTP_10)
    .build()?;
let res = client.execute(request).await?;
println!("Status: {:?}", res.version()); // Status: HTTP/1.0

let client = reqwest::Client::builder().use_rustls_tls().build()?;
let request = client
    .request(reqwest::Method::GET, "https://www.google.com")
    .version(reqwest::Version::HTTP_2)
    .build()?;
let res = client.execute(request).await?;
println!("Status: {:?}", res.version()); // Status: HTTP/2

Resolves #1290

[seanmonstar]

This sounds like a great addition, thank you!

[seanmonstar]

Since http1_only and http2_only are exclusive (they can't both be true), perhaps it'd be best if we made an internal enum to track that?

enum HttpVersionPref {
    Http1,
    Http2,
    All,
}

[seanmonstar]

Naming is always hard. Another option could be http1_only()...

[seanmonstar]

Yesss, let's go! Thanks!