Support using AK from NV

[salrashid123]

GCE VM's encodes a signing key directly at index

const GceAKTemplateNVIndexRSA uint32 = 0x01c10001 // 29425665
const GceAKCertNVIndexRSA uint32 = 0x01c10000 // 29425664

which a customer can recall the final key itself through get-shielded-identity

go-attestation should support loading and using this key directly if the nvindex is specified in AK key acqusition or initialization

eg load and use a key from template here

data, err := tpm2.NVReadEx(t.rwc, tpmutil.Handle(client.GceAKTemplateNVIndexRSA), tpm2.HandleOwner, "", 0)
template, err := tpm2.DecodePublic(data)

// use template to acquire AK and use that

ref handles used on gcp