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.