Add API endpoint to manage pipeline settings - specifically 'Auto cancel pending pipelines'

Zendesk: https://gitlab.zendesk.com/agent/tickets/79402

Customer would like the ability to manage CI/CD pipeline settings via the API. Specifically, they want to be able to disable the 'Auto cancel pending builds' feature. From what I can see, we don't currently support changing any pipeline settings via the API, but we do support changing other project settings.