reqconfig: split business logic into an API class
As part of our effort to create a comprehensive web API alternative to requestctl, we need to extract business logic from the CLI tool and move it to a new API module.
In this series of changes, we introduce the requestctl API module and create an api class that can perform CRUD operations on the objects,
and also most other functions in the CLI module that we'd need elsewhere. While at it, I've uniformed a bit how we operate on haproxy and varnish, when possible.
Bug: T373449