Cache immutable descriptor urls

[roll]

We can detect and cache the url's contents that never changes e.g. from GitHub with commit/ref -- it will speed up working with extensions.

For example camtrapdp:

{
   "name": "...",
   "profile": "https://raw.githubusercontent.com/tdwg/camtrap-dp/<version>/camtrap-dp-profile.json",
   "resources": [
      {
         "name": "deployments",
         "schema": "https://raw.githubusercontent.com/tdwg/camtrap-dp/<version>/deployments-table-schema.json"
      },
      {
         "name": "media",
         "schema": "https://raw.githubusercontent.com/tdwg/camtrap-dp/<version>/media-table-schema.json"
      },
      {
         "name": "observations",
         "schema": "https://raw.githubusercontent.com/tdwg/camtrap-dp/<version>/observations-table-schema.json"
      }
   ]
}