repo-updater: Extend Store with methods for external services

Created by: tsenart

In preparation for the migrations of external services needed in #2025, this change set extends the Store interface with two methods:

1. ListExternalServices(ctx context.Context, kinds ...string) ([]*ExternalService, error)
2. UpsertExternalServices(ctx context.Context, svcs ... *ExternalService) error

All the following changes cascaded from this one and replicate the design of the code that deals with Repos:

1. Introduction of ExternalService and ExternalServices types.
2. Implementation of the ListExternalServices and UpsertExternalServices methods in DBStore and FakeStore.
3. Refactoring of ExternalServicesSourcer to use a Store instead of an InternalAPI.