[CT-2213] [BigQuery] Use new ColumnLevelConstraint structure in bigquery__get_columns_spec_ddl

[MichelleArk]

Core issue: dbt-labs/dbt-core#7066

Refactor bigquery__get_columns_spec_ddl to use the new constraints data structure in favour of the existing constraints and constraints_check. Adapter specific tests should not need to change the existing expected_sql fixture is still the expected result!

Currently, bigquery__get_columns_spec_ddl iterates over the user-provided columns and renders the constraint for each column constraint. It does not render any check constraints because those are passed obtained col['constraints_check']. We should maintain this behaviour - constraints with type: check should not be rendered in the column spec ddl.