Make ETag available in FullMetadata

[shkvoretz]

Hi there,
I can see that file upload response returns an ETag in JSON

{
  "name": "user_storage/23-jobs-donald-duck-has-tried_OMD-feat-1-780x440-1440537749.jpg",

  "bucket": "yanna-ssh.appspot.com",
  "generation": "1654615662670835",
  "metageneration": "1",
  "contentType": "image/jpeg",
  "timeCreated": "2022-06-07T15:27:42.679Z",
  "updated": "2022-06-07T15:27:42.679Z",
  "storageClass": "STANDARD",
  "size": "65729",
  "md5Hash": "NrAPA/++8mfH+5Mv0ksf0A==",
  "contentEncoding": "identity",
  "contentDisposition": "inline; filename*=utf-8''23-jobs-donald-duck-has-tried_OMD-feat-1-780x440-1440537749.jpg",
  "contentLanguage": "en",
  "crc32c": "lXcMLg==",
  "etag": "CPPnwNbTm/gCEAE=",
  "downloadTokens": "ea95b464-3c91-469c-8c6f-ac756ae8b7cc"
}

However, it's not available via FullMetaData which would be really helpful for local file caching.