Add GET /usage endpoint for document count, storage size, and webhook count

[reganwolfrom]

As sites have some limits that can be labeled (currently max number of documents, max storage size, max number of webhooks), it would be helpful to have an endpoint that provides the current stats and maximums (-1 if no maximums); this would also be good in general, as some users have requested.

GET /usage?siteId={siteId}

Response:

{
  "documentCount": 12345,
  "documentCountMax": 50000,
  "totalObjectStorageSize": 678901234, // in bytes
  "totalObjectStorageSizeMax": 2147483648, // in bytes (2 GB)
  "webhookCount": 67,
  "webhookCountMax": 200
}

NOTE: GET /sites/:siteId/configuration uses different properties, maxDocuments, maxContentLengthBytes, and maxWebhooks, so we could do numDocuments instead of documentCount, but ideally we would use objectStorage vs. contentLength here, if possible, to clarify that it is a storage figure, not a response size setting.