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.

app/controllers/repp/v1/certificates/p12_controller.rb

@@ -0,0 +1,29 @@
+module Repp
+  module V1
+    module Certificates
+      class P12Controller < BaseController
+        load_and_authorize_resource param_method: :cert_params
+        
+        THROTTLED_ACTIONS = %i[create].freeze
+        include Shunter::Integration::Throttle
+
+        api :POST, '/repp/v1/certificates/p12'
+        desc 'Generate a P12 certificate'
+        def create
+          api_user_id = cert_params[:api_user_id]
+          render_error(I18n.t('errors.messages.not_found'), :not_found) and return if api_user_id.blank?
+
+          api_user = current_user.registrar.api_users.find(api_user_id)
+          certificate = Certificate.generate_for_api_user(api_user: api_user)
+          render_success(data: { certificate: certificate })
+        end
+
+        private
+
+        def cert_params
+          params.require(:certificate).permit(:api_user_id)
+        end
+      end
+    end
+  end
+end