Support for Model.transform_columns from REST API

[tswast]

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:

python-bigquery/google/cloud/bigquery/model.py

Line 162 in 3e021a4

def training_runs(self) -> Sequence[Dict[str, Any]]:

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.