Follow-up from "feat: smime signed notification emails": support eddsa with curve 25519 using PureEdDSA

The following discussion from !30644 (merged) should be addressed:

• @jritchey started a discussion: (+1 comment)

  If S/MIME Version 4.0 is supported, I'd recommend eddsa with curve25519 using PureEdDSA. Otherwise, it might be limited to RSA