Support for `Model.transform_columns` from REST API

**Is your feature request related to a problem? Please describe.**

When importing a BQ Model into our library, we need to inspect the https://cloud.google.com/bigquery/docs/reference/rest/v2/models#Model.FIELDS.transform_columns property to

**Describe the solution you'd like**

I'd like a `transform_columns` property added to the Model class. This can return a `Sequence[Dict]` similar to what we do for `training_runs`:

https://github.com/googleapis/python-bigquery/blob/3e021a46d387a0e3cb69913a281062fc221bb926/google/cloud/bigquery/model.py#L162

**Describe alternatives you've considered**

Currently, we are working around this issue with the following code:

```
for transform_col in bq_model._properties["transformColumns"]:
```

Also, alternatively we could create a `TransformColumn` class, but given how frequently the BQML API changes and limited capacity for manual updates, I recall we are OK with just exposing the raw-ish REST resources over here.

**Additional context**

This is potentially a good starter project for my apprentices.