Add FetchBundle RPC

FetchBundle will allow updating an existing repository with the objects and refs of a bundle, such as an incremental backup. Right now the only other RPC is CreateRespositoryFromBundle but this requires the repository to not already exist. For incremental backups, there will be a series of bundles that need to be applied to a repository in a specific order.

Ideally in the future we would support bundle-uri, perhaps by accepting a URL and ignoring the stream.