Issue #3: add gateway configuration and validation

src/MxGateway.Server/Configuration/AuthenticationOptions.cs

@@ -0,0 +1,12 @@
+namespace MxGateway.Server.Configuration;
+
+public sealed class AuthenticationOptions
+{
+    public AuthenticationMode Mode { get; init; } = AuthenticationMode.ApiKey;
+
+    public string SqlitePath { get; init; } = @"C:\ProgramData\MxGateway\gateway-auth.db";
+
+    public string PepperSecretName { get; init; } = "MxGateway:ApiKeyPepper";
+
+    public bool RunMigrationsOnStartup { get; init; } = true;
+}