Enhancement: Update the estimator hyperparameters with the placeholder hyperparameters passed to TrainingStep

Currently, it is not possible to update the estimator hyperparameters with the hyperparameters passed to TrainingStep if a Placeholder is used as input. The merging of hyperparameters can only be done if the hyperparameters passed to Training step is a `dict`.

Proposing to add a utility that translates a Placeholder to a usable jsonpath dict (with $). That translated placeholder_dict could be used as hyperparameters input to TrainingStep. With that, merging the constructor and estimator hyperparameters will be possible.
- ex: A Placeholder with schema
```
{
'TrainingInput': {
'B': str,
'C': str
}
}
```
will output the following:
```
{
'B.$': '$$.Execution.Input.B',
'C.$': '$$.Execution.Input.C'
}