amp-story-social-share: share to Instagram story

### Description

By default, when using the system share dialog on a story, it allows you to simply share the link via Instagram.

However, apparently it's possible to provide a `files` list to `navigator.share` to enable posting to Instagram Stories or publishing as a post.

I think that would make sense for stories, maybe by sharing the _poster_ image or dynamically creating a _cover_.

See https://stackoverflow.com/questions/17673156/instagram-share-photo-from-webpage/74802803#74802803


### Alternatives Considered

N/A

### Additional Context

_No response_