Swaps over Nostr

- Separation between SwapManager and its transport:
   Legacy transpport uses http, Nostr uses websockets
 - The transport uses a context to open/close connections.
   This context is not async, because it needs to be called
   from the GUI
 - Swapserver fees values are initialized to None instead
   of 0, so that any attempt to use them before the swap
   manager is initialized will raise an exception.
 - Remove swapserver fees disk caching (swap_pairs file)
 - Regtests use http transport
 - Android uses http transport (until QML is ready)

electrum/lnutil.py

@@ -1520,6 +1520,7 @@ class LnKeyFamily(IntEnum):
     NODE_KEY = 6
     BACKUP_CIPHER = 7 | BIP32_PRIME
     PAYMENT_SECRET_KEY = 8 | BIP32_PRIME
+    NOSTR_KEY = 9 | BIP32_PRIME
 
 
 def generate_keypair(node: BIP32Node, key_family: LnKeyFamily) -> Keypair:
@@ -1528,6 +1529,11 @@ def generate_keypair(node: BIP32Node, key_family: LnKeyFamily) -> Keypair:
     cK = ecc.ECPrivkey(k).get_public_key_bytes()
     return Keypair(cK, k)
 
+def generate_random_keypair() -> Keypair:
+    import secrets
+    k = secrets.token_bytes(32)
+    cK = ecc.ECPrivkey(k).get_public_key_bytes()
+    return Keypair(cK, k)
 
 
 NUM_MAX_HOPS_IN_PAYMENT_PATH = 20