Allow definition of signature value in certificates

[flihp]

The use-case that's driven our feature set till this point could probably be described as general PKI. This has expanded a bit to include use-cases that require the ability to set fields like signatureValue field instead of having one generated by signing the cert with the parent / issuer key. This doesn't make sense if you're trying to build a PKI structure but it does if you're doing something like generating a template for a certificate.