Add searching compara dbs for a gene id at homology endpoint

Created by: Zhicheng-Liu

In EnsemblGenomes we have vertebrate gene ids stored in compara pan homology database. These ids are not loaded into stable_id_lookup database. When a user wants to fetch homologues through a vertebrate gene id, it returns error because the id is not found in stable_id_lookup database.

This change allows searching compara dbs at homology endpoint so that when a vertebrate gene id is not found in the stable_id_lookup database, we continue to search for it in compara dbs, so that the id could be used to fetch homologues through EnsemblGenomes's rest endpoint.

Related to ENSEMBL-5061.