Refactor manifest generation

There's significant duplication between create_manifest and create_oms_manifest. Both functions share manifest creation and cross-reference logic. We could add task to refactor it.
```
enum ManifestFormat {
Standalone,
OMS,
// Future formats...
}

impl ManifestFormat {
fn create(&self, config: ManifestCreationConfig) -> Result<()> {
match self {
Self::Standalone => create_c2pa_manifest(config),
Self::OMS => create_oms_manifest(config),
}
}
}
```

_Originally posted by @sandlbn in https://github.com/IntelLabs/atlas-cli/pull/54#pullrequestreview-3169364368_