Export group and project wikis in offline transfer export

In order to maintain feature parity with direct transfer, offline transfer will need to export project and group wikis to files as well.

Proposed implementation

Wikis on groups and projects are their own git repositories, so we should be able to export them similarly to project repositories:

• Add 'project_wiki' relation to BulkImports::FileTransfer::ProjectConfig and 'group_wiki' relation to BulkImports::FileTransfer::GroupConfig as file reltions.
• In BulkImports::FileExportService, add a case for project and group wikis and call RepositoryBundleExportService with portable.wiki.repository
• Exporting to object storage should be handled in #585537
• Skip these relations if not part of an offline transfer export
• Only export group wikis in EE

In direct transfer, the destination wiki fetches the source wiki as a mirror. Mirroring won't be possible, so that will simply be a limitation of offline transfer.