add support for data-expiry prop on asset tags

To Do: add support for data-expiry prop on asset tags, as to support cache expiry on a per-asset basis

example:
```html

```

The issue provides an HTML img example using data-src, data-key, and data-expiry. Locate the asset-tag handling and existing cache-expiry behavior, then confirm the intended expiry units and verify that the example expires and refreshes one asset independently of others.

Written by the indexing model from the issue text.