Attach citations to paths

Some requirements:
* There can be multiple citations per path
* Citations should be attached to a path in a way that is independent of the identifier so that BioCyc can be updated.
* Citations referenced by doi and information looked up dynamically using the doi.org api.

Example DOI lookup:
```sh
curl -L --header "Accept: application/json" https://doi.org/10.1038/s41598-024-69067-2 | jq .
```