improve manager options workflow

- Break out ManagerOpts type
- Add default configuration values system
- Add WithOpts method
- Rename WithPrefix to WithEnvOverride

config.go

@@ -10,7 +10,29 @@ import (
 	"github.com/spf13/viper"
 )
 
-// Manager is the top-level configuration schema and viper instance.
+// ManagerOpts defines options for the configuration manager.
+type ManagerOpts[T any] struct {
+	// PreLoad is an optional function that is called before loading the
+	// configuration file.
+	PreLoad func(*Manager[T]) error
+
+	// PostLoad is an optional function that is called after loading the
+	// configuration file.
+	PostLoad func(*Manager[T]) error
+
+	// EnvPrefix is an optional environment variable prefix that will be used
+	// to override configuration values from environment variables. If set,
+	// viper will automatically read environment variables with this prefix.
+	EnvPrefix string
+
+	// Defaults is a map of default values for the configuration.
+	Defaults map[string]any
+}
+
+// Manager is the top-level configuration schema and viper instance. Always use
+// the C() method to access the loaded configuration. Always use NewManager to
+// create a new configuration manager. The type parameter T is the configuration
+// schema that will be used to unmarshal the configuration file.
 type Manager[T any] struct {
 	// PreLoad is an optional function that is called before loading the
 	// configuration file.
@@ -49,9 +71,33 @@ func NewManager[T any](configName, configType, configPath string) *Manager[T] {
 	return m
 }
 
-// WithPrefix sets the environment variable prefix for the viper instance. Must
-// be set in order to use environment variables for configuration.
-func (m *Manager[T]) WithPrefix(prefix string) *Manager[T] {
+// WithOpts sets additional options for the Manager.
+func (m *Manager[T]) WithOpts(opts *ManagerOpts[T]) *Manager[T] {
+	if opts == nil {
+		return m
+	}
+
+	if opts.PreLoad != nil {
+		m.PreLoad = opts.PreLoad
+	}
+	if opts.PostLoad != nil {
+		m.PostLoad = opts.PostLoad
+	}
+	if opts.Defaults != nil {
+		for key, value := range opts.Defaults {
+			m.viper.SetDefault(key, value)
+		}
+	}
+	if opts.EnvPrefix != "" {
+		m.WithEnvOverride(opts.EnvPrefix)
+	}
+
+	return m
+}
+
+// WithEnvOverride sets the environment variable prefix for the viper instance
+// and configures viper to automatically read environment variables.
+func (m *Manager[T]) WithEnvOverride(prefix string) *Manager[T] {
 	m.viper.SetEnvPrefix(prefix)
 	m.viper.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
 	m.viper.AutomaticEnv()