Should Certificate be constructable with just a TrustAnchor?

[seanmonstar]

I discussed this briefly I think on Discord, but this feels like a better place. Should reqwest::tls::Certificate have an ability to be constructed from just a trust anchor, instead of requiring a full certificate? The default TLS backend, rustls, only requires the trust anchor information, which is a smaller amount of data, and why webpki-roots exists. But currently, Certificate constructors require the full data, so users need to depend on webpki-root-certs instead.

There's already precedent in reqwest that some of the Certificate constructors might not be usable with a different backend, and they are documented as such. So, we could do the same with Certificate::from_trust_anchor() (or whatever the name is).

It also wouldn't mess with client certificates, because those are reqwest::tls::Identity.