Surface gceInstanceInfo extension attributes for GCE issued AK/EK

[salrashid123]

FR to surface the instance attributes encoded into the x509 issued AK/EK certs on GCE instance:

https://github.com/google/go-tpm-tools/blob/f599e6c6bb64d3c03e9507c9fc12c6dbf4a2f640/server/verify.go#L176

suggestion is having server.VerifyGCECert which directly returns the gceInstanceInfo