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This change isn't intended to have any semantic effect but adds
infrastructure for a caBLE v2 handshake based around Noise NNpsk0. The
implementation has been run against the Noise Explorer implementation[1]
to confirm that it matches the spec.

[1] https://noiseexplorer.com/patterns/NNpsk0/

Change-Id: Iff92bcddfb5b4d07280ef4df5331e471d18bdd18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1752936
Commit-Queue: Adam Langley <agl@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#688339}