Expose negotiated protocol version

[tnull]

The v1.6 upgrade negotiated and stored the server protocol version during connection setup, but omitted the documented public API for reading it.

Add protocol_version() so callers can reuse the negotiated value. Only fall back to server.version when no cached version exists.

[oleonardolima]

utACK f78ed74

Overall it looks good! I pushed a commit fixing the CI, though.

[caarloshenriq]

tACK fadedd9

I wrote this example to validate:

use electrum_client::{Client, Config, ElectrumApi, Socks5Config};
use std::env;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let url = env::args().nth(1).expect("uso: cargo run --example test_protocol_version -- <url> [socks5_addr]");
    let proxy = env::args().nth(2);

    let config = Config::builder()
        .validate_domain(false)
        .socks5(proxy.map(Socks5Config::new))
        .build();

    let client = Client::from_config(&url, config)?;
    println!("{} -> protocol_version = {}", url, client.protocol_version()?);
    Ok(())
}

and tested using these electrum url's

➜  rust-electrum-client git:(pr-213) - cargo run --example test_protocol_version -- "tcp://5dfr3ossuq3kxyc4zsgwqarh37yzlp7b6jc2j7h2gjq6jp7d7zg6fnid.onion:62001" "127.0.0.1:9050"
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.16s
     Running `target/debug/examples/test_protocol_version 'tcp://5dfr3ossuq3kxyc4zsgwqarh37yzlp7b6jc2j7h2gjq6jp7d7zg6fnid.onion:62001' '127.0.0.1:9050'`
tcp://5dfr3ossuq3kxyc4zsgwqarh37yzlp7b6jc2j7h2gjq6jp7d7zg6fnid.onion:62001 -> protocol_version = 1.6
➜  rust-electrum-client git:(pr-213) - cargo run --example test_protocol_version -- "ssl://blackie.c3-soft.com:62002"
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.20s
     Running `target/debug/examples/test_protocol_version 'ssl://blackie.c3-soft.com:62002'`
ssl://blackie.c3-soft.com:62002 -> protocol_version = 1.6
➜  rust-electrum-client git:(pr-213) - cargo run --example test_protocol_version -- "ssl://testnet.aranguren.org:51002"
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.26s
     Running `target/debug/examples/test_protocol_version 'ssl://testnet.aranguren.org:51002'`
ssl://testnet.aranguren.org:51002 -> protocol_version = 1.6
➜  rust-electrum-client git:(pr-213) - cargo run --example test_protocol_version -- "tcp://blackie.c3-soft.com:57005"
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.29s
     Running `target/debug/examples/test_protocol_version 'tcp://blackie.c3-soft.com:57005'`
tcp://blackie.c3-soft.com:57005 -> protocol_version = 1.6