[plugins.json] Add cocoapods-git-private-repo

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

```
{
  "gem": "cocoapods-git-private-repo",
  "name": "CocoaPods Git Private Repo",
  "author": "Nonthawat Srichad",
  "url": "https://github.com/nonth/cocoapods-git-private-repo",
  "description": "CocoaPods plugin for authenticating with private Git repositories using SSH private keys."
}
```