Add a proto comparison macro

[eric846]

Figure out how to make a utility that does this:

  Envoy::ProtobufTypes::MessagePtr empty_config = config_factory.createEmptyConfigProto();
  FakeMetricsPluginConfig expected_config;
  EXPECT_EQ(empty_config->DebugString(), expected_config.DebugString());
  EXPECT_TRUE(Envoy::MessageUtil()(*empty_config, expected_config));

The DebugString version is to make the output clearer for humans. The MessageUtil part is a more complete comparison that may include parts of the proto missing from the DebugString output.

[eric846]

Also consider

  EXPECT_TRUE(::Envoy::Protobuf::util::MessageDifferencer::Equivalent(*empty_config, expected_config));

for the second part, which does the same thing and is clearer.

[eric846]

This is a duplicate of #433.