fix: Implement visit_bytes for the Ed25519PublicKey deserialization

This fixes the deserialization of the SenderData since it switched to
the base64 encoding for serialization of the master key in one of its
variants.

The issue was introduced in 5ff556f6c3.

crates/matrix-sdk-crypto/src/olm/group_sessions/sender_data.rs

@@ -70,13 +70,31 @@ where
         where
             A: de::SeqAccess<'de>,
         {
-            let mut buf = [0u8; 32];
+            let mut buf = [0u8; Ed25519PublicKey::LENGTH];
+
             for (i, item) in buf.iter_mut().enumerate() {
                 *item = seq.next_element()?.ok_or_else(|| de::Error::invalid_length(i, &self))?;
             }
+
             let key = Ed25519PublicKey::from_slice(&buf).map_err(|e| de::Error::custom(&e))?;
+
             Ok(Box::new(key))
         }
+
+        fn visit_bytes<E>(self, v: &[u8]) -> Result<Self::Value, E>
+        where
+            E: de::Error,
+        {
+            if v.len() == Ed25519PublicKey::LENGTH {
+                let mut buf = [0u8; Ed25519PublicKey::LENGTH];
+                buf.copy_from_slice(v);
+
+                let key = Ed25519PublicKey::from_slice(&buf).map_err(|e| de::Error::custom(&e))?;
+                Ok(Box::new(key))
+            } else {
+                Err(de::Error::invalid_length(v.len(), &self))
+            }
+        }
     }
 
     de.deserialize_any(KeyVisitor)