Label encoder opset4 (#17977)
### Description
<!-- Describe your changes. -->
Implements LabelEncoder as per `ai.onnx.ml` opset 4 for the upcoming
ONNX 1.15 release. ~~This currently depends on a new ONNX release
candidate and so is marked as draft in the meantime.~~
### Motivation and Context
Closes https://github.com/microsoft/onnxruntime/issues/17602
