Motivation for this change

I only use bibliography keys as refs, but I use a lot (e.g. for a book I'm half way through, I have nine nodes with the book's ref). When I want to add the ref to a node, at the moment I have to get the book's ref key somehow (e.g. with an embark action from citar, or by copying straight from Ebib), then run ``org-roam-ref-add`, then type '@', and finally insert the ref key and hit RET. This is a heavy workflow for something I do so often.

I imagine other users feel similarly. This PR lets the user set a prompt function for getting refs, which can streamline processes like this.

Personally, I could set this to a simple wrapper over citar-select-ref, though others might do otherwise. This makes the whole process of adding refs much simpler. Leaving the variable at nil retains the current behaviour or just prompting for a string.