Boltcard integration (#5419)

* Boltcard integration

* Add API for boltcard registration

BTCPayServer/Extensions/SettingsRepositoryExtensions.cs

@@ -0,0 +1,32 @@
+#nullable enable
+using BTCPayServer.NTag424;
+using BTCPayServer.Services;
+using static BTCPayServer.Controllers.UIBoltcardController;
+using System.Threading.Tasks;
+
+namespace BTCPayServer;
+public static class SettingsRepositoryExtensions
+{
+    public static async Task<IssuerKey> GetIssuerKey(this SettingsRepository settingsRepository, BTCPayServerEnvironment env)
+    {
+        var settings = await settingsRepository.GetSettingAsync<BoltcardSettings>(nameof(BoltcardSettings));
+        AESKey issuerKey;
+        if (settings?.IssuerKey is byte[] bytes)
+        {
+            issuerKey = new AESKey(bytes);
+        }
+        else
+        {
+            issuerKey = env.CheatMode && env.IsDeveloping ? FixedKey() : AESKey.Random();
+            settings = new BoltcardSettings() { IssuerKey = issuerKey.ToBytes() };
+            await settingsRepository.UpdateSetting(settings, nameof(BoltcardSettings));
+        }
+        return new IssuerKey(issuerKey);
+    }
+    internal static AESKey FixedKey()
+    {
+        byte[] v = new byte[16];
+        v[0] = 1;
+        return new AESKey(v);
+    }
+}