backend: Add Delete() to restic.Backend interface

internal/restic/backend.go

@@ -40,6 +40,9 @@ type Backend interface {
 	// IsNotExist returns true if the error was caused by a non-existing file
 	// in the backend.
 	IsNotExist(err error) bool
+
+	// Delete removes all data in the backend.
+	Delete(ctx context.Context) error
 }
 
 // FileInfo is returned by Stat() and contains information about a file in the