Add support for Sigstore's private key signing flow

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Description

In addition to supporting Sigstore's keyless signing use Fulcio's root key and the OIDC auth flow, we should support Sigstore's private key option for signing and uploading to a Rekor transparency log. This is especially important for heavily regulated industries that are not comfortable with keyless signing options yet, but want to take advantage of the Rekor transparency log component of Sigstore.