[tech] support for Ed25519 in clients

[Kelson Vibber]

 > Do we have a list of which libraries do not include Ed25519?

It looks like it was only recently added to plain Java in JDK 15:
https://seanjmullan.org/blog/2020/10/13/jdk15

That may be why both of the Android clients I tried (Ariane and Deedum)
were unable to connect when I used a default-generated cert from Agate.

FWIW, it also looks like neither Chromium nor Firefox supports Ed25519
signatures
https://security.stackexchange.com/questions/236931/whats-the-deal-with-x25519-support-in-chrome-firefox