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.

config.yaml

@@ -0,0 +1,17 @@
+server:
+  bind_address: ":9090"
+  tls:
+    enabled: false
+    cert_file: "cert.pem"
+    key_file: "key.pem"
+upstream:
+  provider: hetzner
+  hetzner:
+    api_token: "YOUR_HETZNER_API_TOKEN"
+clients:
+  client1:
+    secret: "s3cr3t123"
+    exact:
+      - "home.example.com"
+    wildcard:
+      - "example.net"