Implement caching and optimization for Redner template handling

• Added ormcache to improve performance by caching template retrieval in _get_cached_template.
• Refactored template processing logic:
  • Introduced _to_odoo_template to centralize transformation of external templates into Odoo-compatible format.
  • Simplified template computation in _compute_template and _compute_template_data using cached templates.
• Removed redundant methods (to_cache) and consolidated logic for clarity.

merged