fix(@libp2p/crypto): improve unsupported key type message (#2051)

When an unsupported key type is encountered, but the type is actually missing from the protobuf, don't default the key type in the thrown error to `RSA` because it's not an RSA key, doing so is leads to confusing messages like:

```
invalid or unsupported key type RSA. Must be rsa / ed25519 / secp256k1
```

Instead say that the key type is unknown.

Co-authored-by: Chad Nehemiah <chad.nehemiah94@gmail.com>

packages/crypto/src/keys/index.ts

@@ -69,7 +69,7 @@ export function unmarshalPublicKey (buf: Uint8Array): PublicKey {
     case keysPBM.KeyType.Secp256k1:
       return supportedKeys.secp256k1.unmarshalSecp256k1PublicKey(data)
     default:
-      throw unsupportedKey(decoded.Type ?? 'RSA')
+      throw unsupportedKey(decoded.Type ?? 'unknown')
   }
 }