Pagination fields are missing from assistants list beta API (#571)

curl "https://api.openai.com/v1/assistants?order=desc&limit=20" \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $OPENAI_API_KEY" \
  -H "OpenAI-Beta: assistants=v1"
{
  "object": "list",
  "data": [],
  "first_id": null,
  "last_id": null,
  "has_more": false
}

assistant.go

@@ -52,7 +52,9 @@ type AssistantRequest struct {
 // AssistantsList is a list of assistants.
 type AssistantsList struct {
 	Assistants []Assistant `json:"data"`
-
+	LastID     *string     `json:"last_id"`
+	FirstID    *string     `json:"first_id"`
+	HasMore    bool        `json:"has_more"`
 	httpHeader
 }