Reintroduce alg JWK field for Ed25519 keys (#401)

With https://datatracker.ietf.org/doc/draft-ietf-jose-fully-specified-algorithms/
submitted to IESG for Publication we can add the new "alg" to the
Ed25519 JWK export while accepting both old and new in JWK import.

Author: panva

spec/Overview.html

@@ -10749,6 +10749,14 @@ <h4>Operations</h4>
                             {{DataError}}.
                           </p>
                         </li>
+                        <li>
+                          <p>
+                            If the {{JsonWebKey/alg}} field of |jwk| is present and is
+                            not "`Ed25519`" or "`EdDSA`",
+                            then [= exception/throw =] a
+                            {{DataError}}.
+                          </p>
+                        </li>
                         <li>
                           <p>
                             If |usages| is non-empty and the {{JsonWebKey/use}} field of |jwk| is present and is
@@ -11037,6 +11045,12 @@ <h4>Operations</h4>
                             "`OKP`".
                           </p>
                         </li>
+                        <li>
+                          <p>
+                            Set the `alg` attribute of |jwk| to
+                            "`Ed25519`".
+                          </p>
+                        </li>
                         <li>
                           <p>
                             Set the `crv` attribute of |jwk| to
@@ -15873,7 +15887,8 @@ <h3>Algorithm mappings</h3>
                 <td>
 <pre class=js>
 { kty: "OKP",
-  crv: "Ed25519" }
+  crv: "Ed25519",
+  alg: "Ed25519" }
 </pre>
                 </td>
                 <td>