feat: Initialize Zastava Webhook service with TLS and Authority authentication

- Added Program.cs to set up the web application with Serilog for logging, health check endpoints, and a placeholder admission endpoint.
- Configured Kestrel server to use TLS 1.3 and handle client certificates appropriately.
- Created StellaOps.Zastava.Webhook.csproj with necessary dependencies including Serilog and Polly.
- Documented tasks in TASKS.md for the Zastava Webhook project, outlining current work and exit criteria for each task.

src/StellaOps.Cryptography/DefaultCryptoProvider.cs

@@ -86,6 +86,10 @@ public sealed class DefaultCryptoProvider : ICryptoProvider
     {
         ArgumentNullException.ThrowIfNull(signingKey);
         EnsureSigningSupported(signingKey.AlgorithmId);
+        if (signingKey.Kind != CryptoSigningKeyKind.Ec)
+        {
+            throw new InvalidOperationException($"Provider '{Name}' only accepts EC signing keys.");
+        }
         ValidateSigningKey(signingKey);
 
         signingKeys.AddOrUpdate(signingKey.Reference.KeyId, signingKey, (_, _) => signingKey);