[plugins.json] Add cocoapods-triumph-sdk-plugin

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

```
{
"gem": "cocoapods-triumph-sdk-plugin",
"name": "CocoaPods Triumph SDK Plugin",
"author": "Alex Oakley",
"url": "https://github.com/triumpharcade/cocoapods-triumph-sdk-plugin",
"description": "Helper plugin for installation of TriumphSDK as CocoaPod"
}
```

Open the repository's plugins.json file and compare nearby entries to the requested object. Add the cocoapods-triumph-sdk-plugin entry with the supplied metadata, preserving the file's formatting and valid JSON. Done means the entry appears correctly in plugins.json.

Written by the indexing model from the issue text.