Replace Breez SDK (Greenlight) with Breez Spark SDK (nodeless)

Major changes:
- Built C# bindings for Breez Spark SDK from source using UniFFI
- Created local NuGet package infrastructure (Breez.Sdk.Spark v0.0.1)
- Replaced Breez.Sdk package reference with Breez.Sdk.Spark
- Updated BreezLightningClient to use async Spark SDK API
- Removed Greenlight-specific code (credentials, invite codes)
- Simplified BreezSettings (no more Greenlight fields)
- Updated BreezService for async client initialization
- Cleaned up BreezController (removed certificate upload logic)

Key differences in Spark SDK:
- Nodeless architecture (no Greenlight hosting required)
- Simplified configuration (only mnemonic + API key)
- All async methods (no BlockingBreezServices)
- Different payment flow (PrepareSendPayment + SendPayment)

The plugin now works with Breez's Spark protocol which provides
a self-custodial Lightning experience without infrastructure hosting.

Note: NuGet package must be built from spark-sdk source before use.

Plugins/BTCPayServer.Plugins.Breez/BreezService.cs

@@ -92,7 +92,7 @@ public class BreezService:EventHostedServiceBase
         return settings;
     }
 
-    public  async Task<BreezLightningClient?> Handle(string? storeId, BreezSettings? settings)
+    public async Task<BreezLightningClient?> Handle(string? storeId, BreezSettings? settings)
     {
         if (settings is null)
         {
@@ -105,16 +105,24 @@ public class BreezService:EventHostedServiceBase
         {
             try
             {
-                var network = Network.Main; // _btcPayNetworkProvider.BTC.NBitcoinNetwork;
+                var network = Network.Main;
                 var dir = GetWorkDir(storeId);
                 Directory.CreateDirectory(dir);
                 settings.PaymentKey ??= Guid.NewGuid().ToString();
-                var client = new BreezLightningClient(settings.InviteCode, settings.ApiKey, dir,
-                    network, new Mnemonic(settings.Mnemonic), settings.PaymentKey);
+
+                var client = await BreezLightningClient.Create(
+                    settings.ApiKey,
+                    dir,
+                    network,
+                    new Mnemonic(settings.Mnemonic),
+                    settings.PaymentKey
+                );
+
                 if (storeId is not null)
                 {
                     _clients.AddOrReplace(storeId, client);
                 }
+
                 return client;
             }
             catch (Exception e)