This introduces a new command `cosign signing-config create` to generate a SigningConfig message. This will first be used by our testing infrastructure to create a SigningConfig, and then by users adopting the bundle format in Cosign v2 and eventually as a requirement for v3 when signing with private deployments. Signed-off-by: Hayden B <8418760+haydentherapper@users.noreply.github.com>