[services] Backup/Blob - Add validate method to server write reactor base

Summary:
Depends on D3721

Adding the validate method to the server write reactor base

Test Plan:

cd services
yarn run-backup-service
yarn run-blob-service

Reviewers: palys-swm

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3803