Implement api-php client

[luchaos]

https://github.com/RetroAchievements/retroachievements-api-php

https://github.com/joestrong/retroachievements-composer

https://github.com/spatie/package-skeleton-laravel

[owenvoke]

Do you have any thoughts on using Guzzle or HTTPlug for standardised requests? I'd be happy to help with this.

I've opened PR #2, which refactors to using Guzzle. No pressure to use it, I just thought I'd provide an example of what it could look like.

I was also thinking that the methods could be extracted into concerns/traits to prevent having a list of all methods in the main class. 🤷🏻