Add to_api_repr method to Model

[romainr]

Same as #299 but for models:

AttributeError: 'Model' object has no attribute 'to_api_repr'

Current workaround: using model._properties but object fields like featureColumns https://cloud.google.com/bigquery/docs/reference/rest/v2/models#Model will be missing.

[romainr]

It might be two bugs actually:

• missing to_api_repr
• empty object fields like feature_columns that are part of Model https://cloud.google.com/bigquery/docs/reference/rest/v2/models#Model. list_model is supposed to return a list of Model https://cloud.google.com/bigquery/docs/reference/rest/v2/models/list

[model.feature_columns for model in client.list_models(dataset=dataset)] is empty