ENSCORESW-371: added schema for associated_xrefs
ENSCORESW-371: added schema for associated_xrefs

mjg17/ensembl,muffato/ensembl,muffato/ensembl,willmclaren/ensembl,at7/ensembl,james-monkeyshines/ensembl,willmclaren/ensembl,mjg17/ensembl,Ensembl/ensembl,muffato/ensembl,at7/ensembl,mjg17/ensembl,at7/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,danstaines/ensembl,muffato/ensembl,at7/ensembl,Ensembl/ensembl,Ensembl/ensembl,danstaines/ensembl,james-monkeyshines/ensembl,mjg17/ensembl,james-monkeyshines/ensembl,danstaines/ensembl