sigstore, test: break apart DSSE/artifact sign APIs

[woodruffw]

Per #628: this turns sign(...) into sign_artifact(...) and sign_intoto(...) respectively. In doing so, we're also able to simplify and deduplicate the internals of the Signer class a bit.

[jku]

I think this looks good. I just have docstring and changelog nitpicks.

The changelog rewrite could be left as its own issue: we'll need to review it anyway since you've done so much on this release.

[woodruffw]

The changelog rewrite could be left as its own issue: we'll need to review it anyway since you've done so much on this release.

Yeah, good call. I'll stop updating it with these PRs and file a separate tracker for doing a big update pre-release 🙂