[backup] Update LogItem struct

Summary:
Update the current log item implementation:

• Changed log id to number (the only dynamodb change)
• Used BlobOrDBContent for content and Vec<BlobInfo> for attachments
• Added ensure_size_constraints function
• Implemented From and TryFrom for AttributeValue
• Removed existing db operations for log items (the new improved version will be included in the later diffs)
• Improved constants

Depends on D10405

Test Plan:
Created log item:

LogItem {
  backup_id: "backup_id".to_string(),
  log_id: 124,
  content: BlobOrDBContent::new(vec![0; 2000]),
  attachments: vec![],
};

Called ensure_size_constraints, uploaded it to dynamodb and downloaded it again.

• Checked that blob service wasn't used in this case
• Checked that the downloaded item was the same

Changed the dynamodb item size limit to a smaller value (200 bytes) and repeated the test. This time the content was uploaded to blob service.

Reviewers: bartek, kamil

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10406