RFE: Allow separate GHE server URLs for authorization and code syncing

Created by: mike-r-mclaughlin

Requested by: https://github.com/sourcegraph/accounts/issues/6716 via Slack

Feature request description

Allow separate configurable URLs for GHE code syncing and authn/authz.

Is your feature request related to a problem? If so, please describe.

GHE infrastructure can be complex with geo, repo caches, etc. It will be most efficient to be able to clone repos from a GHE read replica, while handling AuthN and AuthZ from the primary / writeable GHE server.