Load client cert from format *.crt and *.key (native-tls)

[mirecl]

Add new optional fn from_pkcs8_pem for easy load client certificate (*.crt and *.key) for feature native-tls.
Example usage:

use reqwest::blocking::Client;
use reqwest::{Certificate, Identity};
use std::fs;

const URL: &str = "hostname server with mTLS";

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let ca = Certificate::from_pem(&read_file("ca.crt"))?;
    let cert = Identity::from_pkcs8_pem(&read_file("client.crt"), &read_file("client.key"))?;

    let client = Client::builder()
        .add_root_certificate(ca)
        .identity(cert)
        .build()?;

    let status = client.get(URL).send()?.status();

    println!("Status Code: {status}");
    Ok(())
}

fn read_file(path: &str) -> Vec<u8> {
    fs::read(path).unwrap()
}

[mirecl]

@seanmonstar, what do you think about this idea?

[seanmonstar]

Yea, seems like a useful feature to me :)

[mirecl]

@seanmonstar, i fix failed CI for my PR:

• Upgrade minimal version native-tls from 0.2.8 to 0.2.10 (support new function load certificate from_pkcs8);
• Fix test docs.

Pls run again CI check.

[mirecl]

@seanmonstar, tell me when to expect a new release with this feature?

[seanmonstar]

Probably later this week, possible early next week.