Include UniProt protein identifiers for each target gene

**Is your feature request related to a problem? Please describe.**
For the RDF model, I intend to model PPIs from StringDB as `:has_participant :some protein` for each involved protein.

**Describe the solution you'd like**
Include UniProt IDs for each target gene

**Describe alternatives you've considered**
The current solution is a placeholder protein node for each gene, linked to it via `:has_gene_template :gene` but with no real persistent identifier.