[services] Backup - Tools - validate attachment holders

Summary:
Depends on D4080

Adding parseAttachmentHolders function that splits received attachments by the delimiter and performs some sanity checks.

Test Plan: Services can still be built normally. For a full test plan of this functionality, please see D4135

Reviewers: palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4133