feat: add Unknown validation state

[ok-nick]

Adds a new Unknown state for when validation is disabled. Previously, the WellFormed state was added in #1490, but its definition, as well as that of asset state, is unclear. Builds off #1541.

See https://spec.c2pa.org/specifications/specifications/2.2/specs/C2PA_Specification.html#_validation_states.

[codspeed-hq]

CodSpeed Performance Report

Merging #1543 will not alter performance

_{Comparing ok-nick/unknown-validation-state (aadcb02) with main (f6f8998)}

Summary

✅ 16 untouched
⏩ 2 skipped¹

Footnotes

1. 2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[gpeacock]

Does this work? I'm not sure if there will always be no active manifest in validation_results if we did not validate.

[ok-nick]

/// Validation status codes for the ingredient's active manifest. Present if ingredient is a C2PA
/// asset. Not present if the ingredient is not a C2PA asset.
#[serde(rename = "activeManifest", skip_serializing_if = "Option::is_none")]
active_manifest: Option<StatusCodes>,

I think it's a safe assumption that if there's no active_manifest then it hasn't been validated, or at the very least it couldn't be determined. The only problem is that we populate the validation results on a Reader::post_validate, so it's possible that normal validation is disabled but since it was post validated the state shows as invalid (or possibly even valid?).