Add extra headers for OpenAI (#1615)

lua/avante/providers/openai.lua

@@ -278,6 +278,12 @@ function M:parse_curl_args(prompt_opts)
     ["Content-Type"] = "application/json",
   }
 
+  if provider_conf.extra_headers then
+    for key, value in pairs(provider_conf.extra_headers) do
+      headers[key] = value
+    end
+  end
+
   if P.env.require_api_key(provider_conf) then
     local api_key = self.parse_api_key()
     if api_key == nil then