[external IDs] additional possibilities

[Adafede]

• P2840: https://wiki.nci.nih.gov/spaces/NCIDTPdata/pages/155844992/Chemical+Data, but only has SMILES/PubChem CIDs, no InChI(Key)s
• P638: https://www.rcsb.org/docs/programmatic-access/file-download-services
• P11199: https://www.probes-drugs.org/download