db: add index ON explicit_permissions_bitbucket_projects_jobs (project_key, external_service_id, state)

Created by: asdine

We add an index to speed up the duplication of jobs when adding a new job for the same couple project key / code host id. The index can also be used to query jobs by project key in the list API

Related to https://github.com/sourcegraph/sourcegraph/issues/36450

Test plan

sg migration up and undo