reverseproxy: Add tls_curves option to HTTP transport

[bwesterb]

Example Caddyfile

localhost {
    reverse_proxy https://example.com {
        transport http {
            tls_curves secp521r1
        }
    }
}

Long term goal is to be able to configure post-quantum for Caddy -> upstream.

[CLAassistant]

All committers have signed the CLA.

[mholt]

Thank you for this. I noticed it doesn't reject unknown curve names, but maybe that's OK for a niche feature for now. I don't think it will break anything at least.

Since this is a new feature, it will have to wait until 2.9 but we can at least merge it in whenever you're ready.

Did you want to add an error if there's an unrecognized input?

[bwesterb]

Thank you for this. I noticed it doesn't reject unknown curve names, but maybe that's OK for a niche feature for now. I don't think it will break anything at least.

I'm mimicking the behaviour of tls.curve which also doesn't reject unknown curve names.

I can add an error for both if you like?