message: extensions: add explicit type annotation for absl::optional
  When compiling Envoy Mobile locally, I was seeing the following error without this change:
  ```
  external/envoy/source/extensions/transport_sockets/tls/ocsp/asn1_utility.cc:37:23: error: no viable constructor or deduction guide for deduction of template arguments of 'optional'
  return is_present ? absl::optional(data) : absl::nullopt;
  ```
diff: @@ -34,7 +34,7 @@ ParsingResult<absl::optional<CBS>> Asn1Utility::getOptional(CBS& cbs, unsigned t
  return "Failed to parse ASN.1 element tag";
  }
  - return is_present ? absl::optional(data) : absl::nullopt;
  + return is_present ? absl::optional<CBS>(data) : absl::nullopt;
  }
  ParsingResult<std::string> Asn1Utility::parseOid(CBS& cbs) {
  