Refactor: P2PKSecret (#323)

* port changes from dleq branch

* adjust tests

* fix tag serialization

* refactor p2pk and secrets into a new Protocol class

* clean up code

* refactor p2pk types from base

* test second refund pubkey

cashu/wallet/protocols.py

@@ -0,0 +1,16 @@
+from typing import Protocol
+
+from ..core.crypto.secp import PrivateKey
+from ..core.db import Database
+
+
+class SupportsPrivateKey(Protocol):
+    private_key: PrivateKey
+
+
+class SupportsDb(Protocol):
+    db: Database
+
+
+class SupportsKeysets(Protocol):
+    keyset_id: str