storage: download integrity check

### Do not merge
- [ ] integrity tests

Picking up from #299, where we're implementing integrity checks for uploads.
If we already have the `mediaLink` then the request to this url will actually return the MD5 and crc32c hash in the `X-Goog-Hash:` response header so we don't need to re-get metadata. The header will look something like this: 

`X-Goog-Hash: crc32c=D4hAcg==,md5=B3adCJBUBEoAKggSTYiIIw==`