Please consider alternatives to json crate

[scouten-adobe]

This crate depends on json crate which has been unmaintained for more than three years now with no response to active issues or PRs.

We use cargo deny in our CI workflow which is how this issue was discovered. (Details: https://github.com/contentauth/c2pa-rs/actions/runs/6408578517/job/17397889116?pr=317.)

[vstroebel]

#5 Changes the dependency to the jzon crate and makes json support an optional feature.

On the long run using a completely different crate for json support might be a better option.

[scouten-adobe]

@vstroebel thank you. Looking forward to new crate release once that's merged!