Add prune retention planning

internal/config/config.go

@@ -57,6 +57,7 @@ type Destination struct {
 	Links          *Links               `yaml:"links"`
 	State          StatePolicy          `yaml:"state"`
 	Reconciliation ReconciliationPolicy `yaml:"reconciliation"`
+	Retention      RetentionPolicy      `yaml:"retention"`
 	Transfer       TransferPolicy       `yaml:"transfer"`
 }
 
@@ -128,6 +129,16 @@ type StatePolicy struct {
 	Mode string `yaml:"mode"`
 }
 
+type RetentionPolicy struct {
+	Prune PrunePolicy `yaml:"prune"`
+}
+
+type PrunePolicy struct {
+	Enabled    bool      `yaml:"enabled"`
+	OlderThan  *Duration `yaml:"older_than"`
+	KeepLatest *int      `yaml:"keep_latest"`
+}
+
 type TransferPolicy struct {
 	OnDestinationSame  string `yaml:"on_destination_same"`
 	OnDestinationOlder string `yaml:"on_destination_older"`