feat: working version

1. implemented filesystem storage, NATS object storage
and saving to Vault.
2. Test coverage is fine for filesystem and Vault
(and NATS object does not really require extensive tests)

internal/errinternal/errors.go

@@ -0,0 +1,8 @@
+package errinternal
+
+import "errors"
+
+var (
+	ErrInvalidKey = errors.New("storage: invalid key")
+	ErrNotFound   = errors.New("storage: not found")
+)