message: secret: add CertificateValidationContext as a possible Secret.
  No functional changes, only API update.
  *Risk Level*: Low
  *Testing*: bazel test //test/...
  *Docs Changes*: n/a
  *Release Notes*: n/a
diff: @@ -190,9 +190,14 @@ message CommonTlsContext {
  // [#not-implemented-hide:]
  repeated SdsSecretConfig tls_certificate_sds_secret_configs = 6;
  + oneof validation_context_type {
  // How to validate peer certificates.
  CertificateValidationContext validation_context = 3;
  + // [#not-implemented-hide:]
  + SdsSecretConfig validation_context_sds_secret_config = 7;
  + }
  +
  // Supplies the list of ALPN protocols that the listener should expose. In
  // practice this is likely to be set to one of two values (see the
  // :ref:`codec_type <config_http_conn_man_codec_type>` parameter in the HTTP connection
  @@ -262,5 +267,6 @@ message Secret {
  oneof type {
  TlsCertificate tls_certificate = 2;
  TlsSessionTicketKeys session_ticket_keys = 3;
  + CertificateValidationContext validation_context = 4;
  }
  }
  