Add support for ECC

[seannaswell]

Something that is not essential, but possibly worth considering assuming someone has a little time, is updating zotonic_ssl to support ECC.

It is much more efficient than RSA, which means less load on the server (and client), providing faster page load times, and also better security. RSA is still fine, but since Zotonic is engineered to allow better performance on less hardware, using modern cryptography which matches that philosophy, seems to make sense.

A few years ago adoption and support was still an issue, but now both LetsEncrypt and the Erlang SSL library support the use of ECC / ECDSA. https://sectigostore.com/blog/ecdsa-vs-rsa-everything-you-need-to-know/

Unfortunately this is not my area, just wanting to post this here in case assigning this project to someone seems worth pursuing.