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

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The following discussion from gitlab-ce!30644 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