feat: Publish Binary Authorization ContinuousValidationEvent proto.

This is used in the new Continuous Validation feature:
https://cloud.google.com/binary-authorization/docs/overview-cv

Committer: @rbutoi
PiperOrigin-RevId: 373173593

Source-Author: Google APIs <noreply@google.com>
Source-Date: Tue May 11 09:50:37 2021 -0700
Source-Repo: googleapis/googleapis
Source-Sha: bceaea9f824a0d3600cd6a868ea457ac18980049
Source-Link: googleapis/googleapis@bceaea9