[services][backup] AddAttachments 3/4 - main handler logic
Summary:
Implemented the actual AddAttachments endpoint logic. In case of logs, it additionally checks if their size doesn't exceed the size limit. Moving their contents to blob service is implemented in the subsequent diff
Depends on D6205
Test Plan:
The service can be started along with blob service and localstack and the AddAttachments endpoint can now be called for backups.
Testing logs requires adding LogItems manually to DB, as SendLog endpoint is unimplemented yet.
Reviewers: varun, tomek, jon, max
Reviewed By: tomek
Subscribers: ashoat, atul
Differential Revision: https://phab.comm.dev/D6206