Update startup.py, check blink key (#516)

Missing blink key here-added:

for key, value in settings.dict().items():
    if key in [
        "mint_private_key",
        "mint_seed_decryption_key",
        "nostr_private_key",
        "mint_lnbits_key",
        "mint_blink_key",

cashu/mint/startup.py

@@ -28,6 +28,7 @@
         "mint_seed_decryption_key",
         "nostr_private_key",
         "mint_lnbits_key",
+        "mint_blink_key",
         "mint_strike_key",
         "mint_lnd_rest_macaroon",
         "mint_lnd_rest_admin_macaroon",