Add new field to bioentity that is the canonical gene product id for the bioentity

[kltm]

This has GOlr implications: "gene_product" in schema. Will be filled w/ uniprot id, or closest.

This will be very useful for autocomplete and the like.

[kltm]

@cmungall @hdietze

[cmungall]

Let's go with gene_product_id for consistency

[kltm]

"gene_product_id" is consistent if no label is possible (e.g. bioentity_internal_id), otherwise it should be "gene_product", "gene_product_label", etc.