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.

lib/serializers/repp/certificate.rb

@@ -8,16 +8,38 @@ module Serializers
       end
 
       def to_json(obj = certificate)
-        json = obj.as_json.except('csr', 'crt')
+        json = obj.as_json.except('csr', 'crt', 'private_key', 'p12')
         csr = obj.parsed_csr
         crt = obj.parsed_crt
+        p12 = obj.parsed_p12
+        private_key = obj.parsed_private_key
+
+        json[:private_key] = private_key_data(private_key) if private_key
+        json[:p12] = p12_data(obj) if obj.p12.present?
+        json[:expires_at] = obj.expires_at if obj.expires_at.present?
+        
         json[:csr] = csr_data(csr) if csr
         json[:crt] = crt_data(crt) if crt
+        
         json
       end
 
       private
 
+      def private_key_data(key)
+        {
+          body: key.to_pem,
+          type: 'RSA PRIVATE KEY'
+        }
+      end
+
+      def p12_data(obj)
+        {
+          body: obj.p12,
+          type: 'PKCS12'
+        }
+      end
+
       def csr_data(csr)
         {
           version: csr.version,