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/hetzner/hetzner_test.go

@@ -0,0 +1,12 @@
+package hetzner_test
+
+import (
+	"testing"
+
+	pvd "git.cloonar.com/cloonar/updns/internal/provider"
+	"git.cloonar.com/cloonar/updns/internal/provider/hetzner"
+)
+
+func TestNewProviderImplementsInterface(t *testing.T) {
+	var _ pvd.Provider = hetzner.NewProvider("token")
+}