feat: add DER encoding methods and update key type alias in key_utils

Signed-off-by: MonaaEid <monaa_eid@hotmail.com>

Author: MonaaEid

src/hiero_sdk_python/utils/key_utils.py

@@ -33,4 +33,6 @@ def key_to_proto(key: Key | None) -> basic_types_pb2.Key | None:
     if isinstance(key, Key):
         return key.to_proto_key()
 
-    raise TypeError("Key must be of type PrivateKey or PublicKey")
+    raise TypeError(
+        "Key must be of type PrivateKey or PublicKey, or another SDK Key implementation"
+        )