Support totals property on Storage Collections (immediate child only)

[tomcrane]

See Slack thread
https://digirati.slack.com/archives/C06T47XGDV4/p1730990383618139?thread_ts=1730982673.141729&cid=C06T47XGDV4

The actual form of this in the docs is in this commit - https://github.com/dlcs/docs/commit/a624e9f34b8198d11f4168b8289bf8b64e29d0a2

and on the doc site - https://deploy-preview-2--dlcs-docs.netlify.app/api-doc/iiif#paging and below.

   "totals": {
      "childStorageCollections": 4,
      "childIIIFCollections": 0,
      "childManifests": 0,
      "descendantStorageCollections": 12,
      "descendantIIIFCollections": 6,
      "descendantManifests": 3412
   },

Ignore descendant*** for now, just leave these as 0 always.

the child*** properties are the immediate children, so where parent=x.

When this property appears on a IIIF Collection, childStorageCollections must obviously always be 0.