feat: use enum-based artifact sources

[wolfv]

Summary

Adds enum-based artifact source inputs so the main sign/verify functions can accept either artifact bytes/digests or wrapped readers:

• ArtifactSource / AsyncArtifactSource
• reader(...) / async_reader(...) helpers for easy conversion without trait overlap
• existing *_reader methods remain as forwarding compatibility wrappers

Example:

signer.sign(sigstore_sign::reader(file)).await?;
sigstore_verify::verify(sigstore_verify::reader(file), &bundle, &policy, &root)?;

Testing

• git diff --check
• Not run: cargo check (cargo/rustc unavailable in this environment)

[wolfv]

Folded this design into #185 while resolving its merge conflicts, so this stacked PR is no longer needed.