Implement ASSET_MAPPINGS for centralized asset type management

[sandlbn]

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