Include SignatureScheme in the signed payload
Include SignatureScheme in the signed payload

This change adds a SignatureScheme. Validation of the assertion must
ensure that the SignatureScheme hint matches the SignatureScheme in the
signed message. This is to protect against possible signature scheme
substitution attacks.

PiperOrigin-RevId: 202579146

google/asylo,google/asylo,google/asylo,google/asylo,google/asylo,google/asylo