Implement ASSET_MAPPINGS for centralized asset type management

The asset type determination logic is scattered across different functions in src/manifest/utils.rs with hardcoded switch statements. This should be refactored to use a centralized mapping system.

Asset type determination is done through separate functions with hardcoded logic:

- determine_model_type()
- determine_format()
- determine_software_type()
- determine_dataset_type()

**Proposed Solution**

- Populate ASSET_MAPPINGS with file extension to AssetInfo mappings, similar to c2pa lib
- Create get_asset_info() to use case-insensitive lookups
- Refactor the existing determine_* functions to use the centralized mapping
- Remove duplicate/scattered asset type logic