Support for "UseProtoEnumValueNames" option

[Havret]

Feature description

When using the Protobuf.System.Text.Json class to serialize protobuf messages with enum fields, the resulting JSON output does not match the original proto enum value. Instead of using the proto enum value name, the property is being serialized using default System.Text.Json. The same holds for the deserialization. We should add an option to change the way how the enums are serialized/deserialized so it's possible to handle payload that have been serialized using default protobuf to json serialization.

Feature in action

syntax = "proto3";

package MyPackage;

enum TestEnum {
  UNKNOWN = 0;
  FIRST_OPTION = 1;
  SECOND_OPTION = 2;
}

message MyMessage {
  TestEnum enum_field = 1;
}

var msg = new MessageWithEnum
{
    EnumField = TestEnum.FirstOption
};

var jsonSerializerOptions = new JsonSerializerOptions
{
    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
};
jsonSerializerOptions.AddProtobufSupport(options =>
{
    options.UseProtoEnumValueNames = true;
});

var serialized = JsonSerializer.Serialize(msg, jsonSerializerOptions);

{
  "enumField": "FIRST_OPTION"
}

Describe alternatives you've considered

Create custom converters for each enum type in the protobuf contract: This approach requires additional development and maintenance efforts for each enum type and is not a generic solution.