feat: add support for ed25519 in signer

[hitchhooker]

I'm working on a cli that transfers using ed25519 SSH keys and .dot/github identities. however, I couldn't find any support for ed25519 in subxt yet.

I get it that it has been discarded as almost same/lesser than sr25519, but by the looks of it, JAM will introduce shift to using ed25519.

for now, I am using sp_core::{ed25519, crypto::Pair} to handle signing in my project, but it would be great to see support for ed25519 signing in subxt as well.