feat: Add new_mtls for HttpProxy

[cachpachios]

Tiny fix to resolve #783

We expose a new_mtls method for creating a HttpProxy with a client_cert_key to enable mTLS peers.

AFAIK there is currently no other way to set or modify the client_cert_key field. So that small change enables to actually use mTLS HttpPeers, even if the foundations (with the field) been there for a while...

This works well for my own usecase!

Let me know if anyone prefers another approach, i.e a public setter instead of constructor. Or just a different name.
Also, looking at some previous commits it doesnt feel like this sort of changes requires tests to be written. But let me know if thats a wrong assumption.

For anyone looking to use mTLS with a self signed server certificate will you need to modify the PeerOptions too.
Here is an example from my own project:

let mut peer = Box::new(HttpPeer::new_mtls(
    target.address.to_string(),
    target.sni.clone(),
    target.client_cert.clone(),
));
let peer_options = peer.get_mut_peer_options().unwrap();
peer_options.ca = Some(target.ca.clone());