Introduce a Req.RateLimiter module to fix API rate limit issues

lib/discogs/api.ex

@@ -34,6 +34,7 @@ defmodule Discogs.API do
       auth: "Discogs token=#{config.personal_access_token}"
     )
     |> Req.Request.merge_options(config.req_options)
+    |> Req.RateLimiter.attach(name: :discogs, cooldown: config.api_cooldown)
     |> Req.Request.append_request_steps(log_attempt: &log_attempt/1)
     |> Req.Request.append_response_steps(log_error: &log_error/1)
   end