Add API endpoints for fittings and doctrines

[kallama]

Adds API endpoints to

• List fittings
• Get fitting by id
• List doctrines
• Get doctrine by id

Inspiration and code structure taken from https://github.com/dysath/seat-srp

I don't know PHP or Laravel, so I guessed on how to import and use the FittingController class functions inside ApiFittingController.