Implement authenticator based on SPAKE2 implementation in boringssl.

The new authenticator uses SPAKE2 over Curve25519. It will
be enabled in host and client in a separate CL.

BUG=589698

Review URL: https://codereview.chromium.org/1759313002

Cr-Commit-Position: refs/heads/master@{#379972}
8 files changed