Project structure refactor:

- Change package name general_server to gateway
- Changing the structure of directories and packages
- Adding vendor to the project

vendor/go.uber.org/multierr/README.md

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+# multierr [![GoDoc][doc-img]][doc] [![Build Status][ci-img]][ci] [![Coverage Status][cov-img]][cov]
+
+`multierr` allows combining one or more Go `error`s together.
+
+## Features
+
+- **Idiomatic**:
+  multierr follows best practices in Go, and keeps your code idiomatic.
+    - It keeps the underlying error type hidden,
+      allowing you to deal in `error` values exclusively.
+    - It provides APIs to safely append into an error from a `defer` statement.
+- **Performant**:
+  multierr is optimized for performance:
+    - It avoids allocations where possible.
+    - It utilizes slice resizing semantics to optimize common cases
+      like appending into the same error object from a loop.
+- **Interoperable**:
+  multierr interoperates with the Go standard library's error APIs seamlessly:
+    - The `errors.Is` and `errors.As` functions *just work*.
+- **Lightweight**:
+  multierr comes with virtually no dependencies.
+
+## Installation
+
+```bash
+go get -u go.uber.org/multierr@latest
+```
+
+## Status
+
+Stable: No breaking changes will be made before 2.0.
+
+-------------------------------------------------------------------------------
+
+Released under the [MIT License].
+
+[MIT License]: LICENSE.txt
+[doc-img]: https://pkg.go.dev/badge/go.uber.org/multierr
+[doc]: https://pkg.go.dev/go.uber.org/multierr
+[ci-img]: https://github.com/uber-go/multierr/actions/workflows/go.yml/badge.svg
+[cov-img]: https://codecov.io/gh/uber-go/multierr/branch/master/graph/badge.svg
+[ci]: https://github.com/uber-go/multierr/actions/workflows/go.yml
+[cov]: https://codecov.io/gh/uber-go/multierr