feat(config): add yaml config with generic env override

Adds internal/config package using gopkg.in/yaml.v3 and reflection for generic environment variable overrides. Updates main.go to load config and set log level. Adds tests and example config.

config/config.yaml

@@ -0,0 +1,7 @@
+# Application Configuration
+# This file contains the default configuration for the application.
+# Values can be overridden by environment variables with the prefix "APP_".
+# Example: APP_LOG_LEVEL=debug
+
+# Log Level: debug, info, warn, error
+log_level: "info"