[Feature Request]: 目录

目录的增删查改，包括 `我建立的目录` 和 `某用户的收藏目录`

目前已经存在
[/v0/indices/{index_id}](https://bangumi.github.io/api/#/%E7%9B%AE%E5%BD%95/getIndexById)
[/v0/indices/{index_id}/subjects](https://bangumi.github.io/api/#/%E7%9B%AE%E5%BD%95/getIndexSubjectsByIndexId)

## 已实现 #234
- [x] 新建目录 POST `/indices`
- [x] 修改 PUT `/indices/{index_id}`
- [x] 增加内容 POST `/indices/{index_id}/subjects`
- [x] 修改内容 PUT `/indices/{index_id}/subjects/`
- [x] 删除内容 DELETE `/indices/{index_id}/subjects/`

## 待实现
- [x] 收藏目录 POST `/indices/{index_id}/collect`
- [x] 取消收藏 DELETE `/indices/{index_id}/collect`
- [ ] 用户的目录 GET `/users/{username}/indices`
- [ ] 用户的收藏目录 GET `/users/{username}/indices/collect`

## 暂定不添加
- [ ] 删除 DELETE `/indices/{index_id}`

Start by reading the existing index endpoints `/v0/indices/{index_id}` and `/v0/indices/{index_id}/subjects`, then trace the implementation of the already completed index and collection routes. The remaining work is the two user endpoints: `/users/{username}/indices` and `/users/{username}/indices/collect`; it is done when both return the requested user's created and collected indexes.

Written by the indexing model from the issue text.