Add validate_msg and test

libp2p/pubsub/validators.py

@@ -0,0 +1,9 @@
+# FIXME: Replace the type of `pubkey` with a custom type `Pubkey`
+def signature_validator(pubkey: bytes, msg: bytes) -> bool:
+    """
+    Verify the message against the given public key.
+    :param pubkey: the public key which signs the message.
+    :param msg: the message signed.
+    """
+    # TODO: Implement the signature validation
+    return True