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.go

@@ -0,0 +1,8 @@
+package provider
+
+import "context"
+
+// Provider defines the interface for updating DNS records.
+type Provider interface {
+	UpdateRecord(ctx context.Context, domain, ip string) error
+}