[CT-2221] [BigQuery] Add model-level constraints to get_columns_spec_ddl

[MichelleArk]

Core issue: dbt-labs/dbt-core#6754

BigQuery only supports the not_null constraint

constraint type	support
not_null	enforced
primary_key	not supported
foreign_key	not supported
unique	not supported
check	not supported

BigQuery does not support model-level constraints, so no need to render them in the ddl. We'll just need to ensure tests are passing - skipping any model-level constraint tests inherited from the base adapter tests class.

Constraint docs: https://cloud.google.com/bigquery/docs/reference/standard-sql/data-definition-language