Update reference server session binding logic
Update reference server session binding logic

This commit updates the reference server implementation's handling of
attestation session binding to match changes made to the client in
commit 4e85440e. The attestation prefix is now only used as the nonce
for the attestation, so it no longer needs to be skipped when reading
and unmarshaling the attestation evidence records.

This fix was tested by running the securesession binary against the
server binary on a Confidential VM that produces an attestation, and
verifying the secure session is successfully established.

PiperOrigin-RevId: 398823961
Change-Id: If92af35bcdabaef866ac709f78c5d236162ebf6b

GoogleCloudPlatform/stet