Introduce internal interface for `ValuePropBackend`

[neNasko1]

Duplicate of #201 because of missing access to the repo

Since we currently "bless" the reference and onnxruntime implementations as the only way to propagate value internally, this PR refactors a bit the internals to allow a better interface for "custom" execution providers(i.e. @cbourjau rust runtime, onnx-mlir...). Consult the tests for a POC which could also be used to register custom opsets as well.

Checklist

• [X] Added a CHANGELOG.rst entry