[services][backup] AddAttachments 4/4 - moving to blob

Summary:
This implements the last part of the AddAttachments endpoint - moving log item storage to blob service when it's too large.

Depends on D6206

Test Plan:

1. Manually created large log item, ensured that it's slightly less than 400kb.
2. Called AddAttachments providing a long attachments holder string.
3. Ensured Blob service got called and the log was uploaded & database entity was updated.

The backup service integration tests cover this case, but this needs remaining endpoints to be implemented.

Reviewers: varun, tomek, jon, max

Reviewed By: tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6207