Bitbucket Cloud external service doesn't provide "exclude" config option

Created by: dadlerj

A user at https://app.hubspot.com/contacts/2762526/company/2478384295 just emailed and asked if we provided any way to exclude certain repos using a Bitbucket Cloud external service, like we do for Bitbucket Server, GitHub, etc...

Looks like this never got added: https://docs.sourcegraph.com/admin/external_service/bitbucket_cloud#admin-external-service-bitbucket-cloud-schema-json