a0c49755c4
Adds a `BackupResource` abstract class to be extended by more specific resource handlers and a `BackupResources` module which resources can be registered with. The BackupResource base includes helpers to get the size of files and directories. All registed resources will be provided to the `BackupService` constructor for it instantiate them. Differential Revision: https://phabricator.services.mozilla.com/D203795