[TECH ED] Mailing list API

### Link to the coursework

https://github.com/CodeYourFuture/Module-Servers/tree/main/mailing-list-api

### Why are we doing this?

In this challenge you will take all your knowledge of APIs and backend thus far to build an API that can process a full range of different HTTP requests.

### Maximum time in hours

12

### How to get help

Share your blockers in your class channel
https://curriculum.codeyourfuture.io/guides/getting-help/asking-questions/

### How to submit

1. Fork to your Github account.
2. Make a branch for this project.
3. Make regular small commits in this branch with clear messages.
4. When you are ready, open a PR to the CYF repo, following the instructions in the PR template.
5. Connect your PR to this ticket by writing #89 in your message

```mermaid
gitGraph
commit id: "start"
branch feature/mailing-list-api
commit id: "update gitignore"
commit id: "skeleton code with requirements"
commit id: "tests for route lists"
commit id: "GET, PUT"
commit id: "DELETE"
commit id: "revising tests, small fixes, listed"
checkout main
merge feature/mailing-list-api
```

リンクされている mailing-list-api の coursework から始め、その要件と提出手順を確認してください。そこに記載されている API を構築して必要な HTTP リクエストを処理し、その後、記載されている動作を検証してから issue #89 に関連付けた PR を作成してください。

索引モデルが issue の本文から書いたものです。