Expose arrow_json::writer::encoder::make_encoder_impl() for re-use in custom encoders

[eldargab]

In our project we have a set of JSON serialization rules not covered by arrow_json.

For example, there are binary columns that already contain a valid JSON that needs to be just embedded "as is" into the output stream. We also have cases when we'd like to serialize some StructArrays in non-canonical way without any prior transformation.

Public make_encoder_impl() would allow us to re-use existing arrow encoders in our serializers.

[tustvold]

I think the encoder details are the kind of thing that would be good to keep as a private implementation detail.

Perhaps we might provide a way to indicate that a given StringArray should be encoded as-is without escaping?