Support output overrides

### What problem are you facing?

Add the ability to override the merged context with fields coming from the XR.
For example this is useful in scenarios where we _load_ fields from an `EnvironmentConfig` such as `region` and we want to transparently override this to all context users (subsequent pipeline steps) with a value from the XR (e.g. under `spec.parameters.region`).
Even though this could be done by adding an additional step to mutate the context, doing this from `function-environment-config` would be useful and saves an extra step in the pipeline.

### How could this Function help solve your problem?

Implement a way to copy a value from an XR path to the resulting (merged) context.