Address ENG-10731.
I had to use TransactWriteItems because it's the only DDB endpoint capable of updating multiple items in batch, without entirely overwriting.
When this function fails, the fetch-sizes endpoint still works, simply DDB won't be updated.

Depends on D14646

On staging:

1. Found a blob without the blob_size attribute in DDB
2. Called the blob sizes endpoint, which retrieved the size from S3
3. Checked DDB again. The size was now stored in DDB.