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/config/config.go

@@ -0,0 +1,62 @@
+package config
+
+import (
+	"fmt"
+
+	"github.com/spf13/viper"
+)
+
+type ClientConfig struct {
+	Secret   string   `mapstructure:"secret"`
+	Exact    []string `mapstructure:"exact"`
+	Wildcard []string `mapstructure:"wildcard"`
+}
+
+type ServerConfig struct {
+	BindAddress string    `mapstructure:"bind_address"`
+	TLS         TLSConfig `mapstructure:"tls"`
+}
+
+type TLSConfig struct {
+	Enabled  bool   `mapstructure:"enabled"`
+	CertFile string `mapstructure:"cert_file"`
+	KeyFile  string `mapstructure:"key_file"`
+}
+
+type HetznerConfig struct {
+	APIToken string `mapstructure:"api_token"`
+}
+
+type UpstreamConfig struct {
+	Provider string        `mapstructure:"provider"`
+	Hetzner  HetznerConfig `mapstructure:"hetzner"`
+}
+
+type Config struct {
+	Server   ServerConfig            `mapstructure:"server"`
+	Upstream UpstreamConfig          `mapstructure:"upstream"`
+	Clients  map[string]ClientConfig `mapstructure:"clients"`
+}
+
+// LoadConfig reads the file at path (yaml, json, toml) into Config and validates it.
+func LoadConfig(path string) (*Config, error) {
+	v := viper.New()
+	v.SetConfigFile(path)
+
+	if err := v.ReadInConfig(); err != nil {
+		return nil, fmt.Errorf("reading config: %w", err)
+	}
+
+	var cfg Config
+	if err := v.Unmarshal(&cfg); err != nil {
+		return nil, fmt.Errorf("parsing config: %w", err)
+	}
+
+	for name, client := range cfg.Clients {
+		if len(client.Exact) == 0 && len(client.Wildcard) == 0 {
+			return nil, fmt.Errorf("client %q must have at least one of exact or wildcard", name)
+		}
+	}
+
+	return &cfg, nil
+}