Store the class metadata of fields in the class, to improve preformance

Cached data include,
- lookup table between groups and fields of "oneof" fields
- default value creator of each field
- type hint of each field

betterproto/templates/template.py

@@ -1,7 +1,6 @@
 # Generated by the protocol buffer compiler.  DO NOT EDIT!
 # sources: {{ ', '.join(description.files) }}
 # plugin: python-betterproto
-from dataclasses import dataclass
 {% if description.datetime_imports %}
 from datetime import {% for i in description.datetime_imports %}{{ i }}{% if not loop.last %}, {% endif %}{% endfor %}
 
@@ -38,7 +37,7 @@ class {{ enum.py_name }}(betterproto.Enum):
 {% endfor %}
 {% endif %}
 {% for message in description.messages %}
-@dataclass
+@betterproto.protoclass
 class {{ message.py_name }}(betterproto.Message):
     {% if message.comment %}
 {{ message.comment }}