support ed25519-sha256 for DKIM

[slingamn]

1. Implement support ed25519-sha256 toorop/go-dkim#17
2. Roll it out in oragono

[streaps]

Link to the non-draft RFC:
https://tools.ietf.org/html/rfc8463

[slingamn]

We can probably just switch to https://pkg.go.dev/github.com/emersion/go-msgauth/dkim

[slingamn]

This works now, but major email providers don't support it: https://www.uriports.com/blog/dkim-ed25519-adoption/

so there is not much point in enabling this; if you publish a TXT record for a weak fallback key, then anyone can sign with just that key.