Refactor manifest generation

[marcelamelara]

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 #54 (review)