A Req request step that signs outgoing requests per RFC 9421.

Req.new(base_url: "https://api.example.com")
|> MessageSignatures.Req.attach(
  key: {:ed25519, private_key},
  key_id: "my-key-2026",
  components: ["@method", "@authority", "@path", "content-digest"]
)
|> Req.post!(url: "/payments", body: payload)

When http_digest is available, the default digest: true attaches its HTTPDigest.Req step before this signing step. This lets the signature cover the resulting Content-Digest field.