[plugins.json] Add cocoapods-monorepo

Please add the following entry to the `plugins.json` file:

```
{
"gem": "cocoapods-monorepo",
"name": "CocoaPods Monorepo",
"author": "menttofly",
"url": "https://github.com/menttofly/cocoapods-monorepo.git",
"description": "`cocoapods monorepo` will make the dependencies that under specified directory into development pods."
}
```

Open plugins.json and compare nearby entries to the requested cocoapods-monorepo object. Add the entry with the supplied gem, name, author, URL, and description, then verify that the JSON remains valid and the plugin appears in the catalog.

Written by the indexing model from the issue text.