certificate: accept pem format

[SSebo]

I found quinn::Certificate missing a from_pem function when I use pem format certificates, just add it.

[djc]

Seems okay to me (modulo the formatting nit from CI -- ignore the clippy warning). @Ralith, any thoughts?

[Ralith]

I don't have a great grasp on the pem format itself; are there times when it'd be more appropriate to use this over CertificateChain::from_der?

[stammw]

Pem seems easier to handle from an ops point of view as most providers (i know) use this format. This PR simplify user's life by saving them the converting work IMHO.

[Ralith]

Er, sorry, that was a typo. I meant to ask whether there are situations where you can't use CertificateChain::from_pem. There is no CertificateChain::from_der.

[SSebo]

I found der format in server.rs example is:

let key = quinn::PrivateKey::from_der(&key)?;
let cert = quinn::Certificate::from_der(&cert)?;
server_config.certificate(quinn::CertificateChain::from_certs(vec![cert]), key)?;

I followed the example, think maybe pem format should be this way:

let key = quinn::PrivateKey::from_pem(&key)?;
let cert = quinn::Certificate::from_pem(&cert)?;
server_config.certificate(quinn::CertificateChain::from_certs(vec![cert]), key)?;

after I read @Ralith 's comment, I found following way is working as well:

let key = quinn::PrivateKey::from_pem(&key)?;
server_config.certificate(quinn::CertificateChain::from_pem(&cert.to_vec())?, key)?;

In my case, client.rs need this Certificate::from_pem:

client_config.add_certificate_authority(quinn::Certificate::from_pem(&cert)?)?;

[Ralith]

Seems reasonable to me.

[djc]

@SSebo please fix up the formatting, then we can merge it! Thanks!