feat: Implement configuration management and DNS provider integration

- Added configuration management using Viper in internal/config/config.go
- Implemented ClientConfig, ServerConfig, TLSConfig, HetznerConfig, UpstreamConfig, and main Config struct.
- Created LoadConfig function to read and validate configuration files.
- Developed Hetzner DNS provider in internal/provider/hetzner/hetzner.go with methods for updating DNS records.
- Added comprehensive unit tests for configuration loading and Hetzner provider functionality.
- Established HTTP server with metrics and update endpoint in internal/server/server.go.
- Implemented request handling, authorization, and error management in the server.
- Created integration tests for the Hetzner provider API interactions.
- Removed legacy dynamic DNS integration tests in favor of the new API-based approach.

internal/provider/provider_test.go

@@ -0,0 +1,22 @@
+package provider_test
+
+import (
+	"context"
+	"testing"
+
+	"git.cloonar.com/cloonar/updns/internal/provider"
+)
+
+// mockProvider is a dummy implementation for testing the Provider interface.
+type mockProvider struct{}
+
+func (m *mockProvider) UpdateRecord(ctx context.Context, domain, ip string) error {
+	return nil
+}
+
+func TestProviderInterfaceCompliance(t *testing.T) {
+	var p provider.Provider = &mockProvider{}
+	if err := p.UpdateRecord(context.Background(), "example.com", "1.2.3.4"); err != nil {
+		t.Errorf("expected no error, got %v", err)
+	}
+}