feat(transport): added support for EC keys

[YuJuncen]

Signed-off-by: Yu Juncen yu745514916@live.com

Motivation

This PR tries to fix #1143. Which caused tonic reports error when using EC keys as client keys.

Solution

The solution has been described in the issue. Copy & paste it here:

By reading the code, I have noticed that we are going to extract key via applying pkcs8_private_keys and rsa_private_keys. It seems rustls-pemfile supports SEC1 EC keys however doesn't provide something like ec_private_keys (!).
Perhaps we can use read_once direcly, so we only need to iterate the key file once and can extract the EC keys:

fn load_rustls_private_key(
    mut cursor: std::io::Cursor<&[u8]>,
) -> Result<PrivateKey, crate::Error> {
    while let Ok(Some(item)) = rustls_pemfile::read_one(&mut cursor) {
        match item {
            rustls_pemfile::Item::RSAKey(key)
            | rustls_pemfile::Item::PKCS8Key(key)
            | rustls_pemfile::Item::ECKey(key) => return Ok(PrivateKey(key)),
            _ => continue,
        }
    }

    // Otherwise we have a Private Key parsing problem
    Err(Box::new(TlsError::PrivateKeyParseError))
}

[LucioFranco]

Why pub(crate), I don't see this used outside of the crate or am I going crazy?

[YuJuncen]

I want to use it in the tests module, if I remove this rustc would blame me load_rustls_private_key is a private function... Are there some better solutions?

Well, I guess pub(super) is what we need?