[services] Backup - Add Attachment Reactor Logic

Summary:
Depends on D4134

Adding AddAttachments logic. This is a Unary Reactor which means it goes only for one cycle, the response is empty, so it effectively just receives the data on the server and then returns a status.

logID is optional. If it's an empty string, the attachments are appended to the backup item.

Test Plan:

• terminal 1

cd services
yarn run-blob-service-dev-mode

• terminal 2

cd services
yarn run-backup-service-dev-mode

• terminal 3

git clone https://github.com/karol-bisztyga/grpc-playground.git
cd grpc-playground
git checkout backup-async
./build.sh
./cmake/build/bin/client

• in terminal 3 create a new backup with n
• check the database and see the empty string in attachmentHolders in the newly created item
• add new attachments: in terminal 3 use b
• check the database again and see new attachments in attachmentHolders in the newly created item
• do a similar thing for the logs: first, use l and then k

Reviewers: palys-swm

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D4135