This aligns the DLEQ proof portion of TRUST_TOKEN_pst_v1_voprf
with draft-irtf-cfrg-voprf-21. The blind and finalize operations
still differ. Additionally, as VOPRF doesn't include batched
issuance, the issuance process around the DLEQ proof is adapted
from draft-robert-privacypass-batched-tokens-01.
Bug: chromium:1414562
Change-Id: If1c6de0f92089a826968a57279ae598ccf89ca3e
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/58906
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
4b6d950d89