feat: improve certificate download extensions

Update certificate download functionality to use appropriate file extensions:
- Use .p12 extension for PKCS#12 files
- Keep .pem extension for PEM-encoded files (CSR, CRT, private key)

This change ensures that downloaded certificate files have the correct extension based on their format, making it easier for users to identify and use the files correctly.

config/routes.rb

@@ -118,7 +118,11 @@ Rails.application.routes.draw do
         end
       end
       resources :white_ips, only: %i[index show update create destroy]
-      resources :certificates, only: %i[create]
+      resources :certificates, only: %i[create] do
+        scope module: :certificates do
+          post 'p12', to: 'p12#create', on: :collection
+        end
+      end
       namespace :registrar do
         resources :notifications, only: %i[index show update] do
           collection do