feat: Add a factory method to the Reader class

[tmathern]

Changes in this pull request

Add a factory method to the Reader class:

• if there is JUMBF (c2pa/manifest) data to read, from_asset will return a Reader instance
• if there is no JUMBF (c2pa/manifest) data to read, returns None instead of throwing

Builds on top of #213 which improves error handling (and enables to write shorter code here).

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented.
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment.