Support Access of Dictionary Items in matrix-stages

When I work with matrix-stages I would like to use further parameters for each item, specified in the params.yaml
A prototpy-stage would look like

# dvc.yaml
stages:
  eval:
    matrix:
      model: ${models}
      testset: ${testsets}
   cmd: python src/eval_script.py --model ${item.model} --evalset ${item.evalset} --model-family ${item.model.family}
...

And the params.yaml would be:

# params.yaml
models:
  m1:
    family: b21
  m2: 
    family: x22
      
testsets: [evalA, evalB]

This is similar to foreach-stages, where you can access properties by calling ${item.property}

Start by tracing how matrix-stages expands the model and testset entries from dvc.yaml and params.yaml, then compare that path with foreach-stages' item.property handling. Done means dictionary-valued matrix items support expressions such as ${item.model.family}, with coverage for the example behavior.