Add ED25519 ctor for just pub key part.

Author: darinkes

Diff for: src/Renci.SshNet/Security/Cryptography/ED25519Key.cs

@@ -99,6 +99,15 @@ public ED25519Key()
         {
         }
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ED25519Key"/> class.
+        /// </summary>
+        /// <param name="pk">pk data.</param>
+        public ED25519Key(byte[] pk)
+        {
+            publicKey = pk.TrimLeadingZeros().Pad(Ed25519.PublicKeySizeInBytes);
+        }
+
         /// <summary>
         /// Initializes a new instance of the <see cref="ED25519Key"/> class.
         /// </summary>