Add collection fetching module

[akkoma] / test / fixtures / collections / unordered_page_reference.json

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "summary": "Sally's recent activities",
  "type": "Collection",
  "id": "https://example.com/collection/unordered_page_reference",
  "totalItems": 10,
  "first": "https://example.com/collection/unordered_page_reference?page=1"
}