feat: define DigitalSourceType as enum

[ok-nick]

Changes in this pull request

Define Actions::source_type as an enum so user's no longer need to manually specify the entire URL.

Questions

• Do we want to provide the Other(String) variant or ensure they are only what we support?
  • This has the flexibility that it supports future c2pa versions but it can allow for incorrect URLs.
• A side-effect of this implementation is that we allow digital source types to be deserialized as, e.g. digsrctype:digitalCapture in addition to the URL, which is technically incorrect according to the spec but offers a much more convenient JSON manifest/settings definition.

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.

[ok-nick]

The failing test actually caught 2 issues: one that I should've untagged the DigitalSourceType::Other variant and second, our CAWG assets have invalid IPTC source type URLs.

[gpeacock]

Thanks, It's great having this.

[ok-nick]

@gpeacock I'm not sure if it is a mistake in the spec, but the C2PA empty digital source type is defined as part of the digsrctype namspace here:
https://c2pa.org/digitalsourcetype/empty/

[gpeacock]

This looks good. I just have one more question about using the digsrctype prefixes. I'm not sure we need those in the context in which we would be using this:

"digtialSourceType": "digsrctype:digitalCapture".
vs
"digtialSourceType": "digitalCapture".