Add support for ECDSA server certificates

[sysfu]

I've been switching over to ECDSA TLS certificates as they need to be renewed, however it appears that relayd currently lacks support for this technology.

The reasons one might want to migrate to ECDSA certs are presented in this Cloudflare article

If it's not too much of a headache to code, please add support for ECDSA certs.

[vg]

I second this request. Please add support for ECDSA certs. Let's Encrypt is soon planning to also issue ECDSA certs.