Create service to write metadata.json

Because offline transfer imports can't query the source destination to get instance details, offline transfer exports need to write migration metadata to a new file, metatada.json.

Proposal

Create a new service that gathers instance data, export file structure in object storage, and user mapping information. This service should be executed once all relation exports have finished. See https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/offline_direct_transfer_migrations/#import-metadata-file-structure for attributes that need to be stored in the file.