Transformer: Add TranscodingTransformer.

Temporary file copy of Transformer, which uses
TransformerTranscodingVideoRenderer instead of
TransformerMuxingVideoRenderer to transform files. This allows devs to test
transcoding more easily using the Transformer demo, while external engineers
continue to see the completely working Muxing-based transformer.

In the future, this will replace the Transformer class.

PiperOrigin-RevId: 401020893