[C#] Support snake_case keys in JsonFormatter

[rok-povsic]

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What language does this apply to? proto3, C#.

Describe the problem you are trying to solve.

I'd like to serialize a Protobuf message to JSON using JsonFormatter, where keys named using snake_case are kept, instead of being converted into camelCase.

Describe the solution you'd like

I would like the keys to be kept as snake_case.

Describe alternatives you've considered

The alternative I have is taking the output string, parse the JSON, and recursively iterate through all the keys to convert camelCase back to snake_case.

Additional context

In Protobuf Python, for example, this is already possible using the preserving_proto_field_name field.

[anandolee]

We could add an option is json fotmat setting and a method public Settings WithPreserveFieldName(bool perserveFieldName) => new Settings(formatDefaultValues, TypeRegistry, FormatEnumsAsIntegers, perserveFieldName)

https://github.com/protocolbuffers/protobuf/blob/master/csharp/src/Google.Protobuf/JsonFormatter.cs#L781

Are you happy to create a PR and assign to me for review?

[rok-povsic]

@anandolee It seems like it's not trivial to add this option because ToJsonName is static and cannot access the JsonFormatter.Settings object.

From what I understand, ToJsonName gets called in the FieldDescriptor constructor and adding this option would require changing multiple classes' constructors, outside of JsonFormatter.

Do you see some other, easier solution?

[anandolee]

ToJsonName() is called in some helper APIs or generated code.

Users can create their own JsonFormatter() with settings and then call formatter.Format(message). For example: https://github.com/protocolbuffers/protobuf/blob/master/csharp/src/Google.Protobuf.Test/JsonFormatterTest.cs#L90

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