Add table_constraints field to Table model

[dimkonko]

Is your feature request related to a problem? Please describe.
I'd like to get the Table.table_constraints value returned from the API. The model is described in the API reference: https://cloud.google.com/bigquery/docs/reference/rest/v2/tables#tableconstraints.
This will allow to proceed with googleapis/google-cloud-python#15788

Describe the solution you'd like
I'd like to add table_constraints field to Table model and serialize the value from the API response.

Describe alternatives you've considered
For python-bigquery-sqlalchemy we can get the values from the Table instance _properties directly. But the serialized value is better and allows to access the data using public interface.

Additional context
N/A