Add option to modify HTTP pool size

[akkoma] / test / fixtures / users_mock / masto_closed_following.json

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "id": "http://localhost:4001/users/masto_closed/following",
  "type": "OrderedCollection",
  "totalItems": 152,
  "first": "http://localhost:4001/users/masto_closed/following?page=1"
}