[FR] relative buffer-local path to bib for org-cite biblatex

[Panayotis Manganaris <panos.manganaris@gmail.com>]

Hello, I just noticed that adding a local bibliography for use by the 
biblatex cite processor gets exported to latex as an absolute path.

`#+bibliography: main.bib`  exports to 
`\addbibresource{/home/me/etc/main.bib}`

I would like a setting to allow this to be a relative path if possible.

I usually generate a local bibliography when creating a manuscript 
repository intended to share. I'd rather have the local bib be defined 
relative to the main file in that case.

looking at `oc.el` I see no way to make this setting. but I image it'd 
be popular.

```elisp

(defun org-cite-list-bibliography-files ()
   "List all bibliography files defined in the buffer."
   (delete-dups
    (append (mapcar (lambda (value)
              (pcase value
                (`(,f . ,d)
                         (expand-file-name (org-strip-quotes f) d))))
            (pcase (org-collect-keywords
                            '("BIBLIOGRAPHY") nil '("BIBLIOGRAPHY"))
              (`(("BIBLIOGRAPHY" . ,pairs)) pairs)))
        org-cite-global-bibliography)))
```

I'll have to learn elisp soon. Thanks for maintaining my favorite tools! 
Best,

Panos