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

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()](https://github.com/apache/arrow-rs/blob/119db5cdc64d52955a24fc08888f10db69019062/arrow-json/src/writer/encoder.rs#L53) would allow us to re-use existing arrow encoders in our serializers.

Start in arrow-json/src/writer/encoder.rs at make_encoder_impl() and inspect how the existing encoder construction is exposed. Confirm the API needed by custom serializers and ensure the function can be reused outside its current scope without changing existing encoder behavior.

Written by the indexing model from the issue text.