[ADAP-351] [BigQuery] Generate order-agnostic constraints ddl when contract: true

[MichelleArk]

Core issue: dbt-labs/dbt-core#7064

Wrap create_table_as user sql in schema-ordered select statement for models with contract: true

{% macro <adapter>__create_table_as(temporary, relation, sql) -%}
    ... 
    {% if config.get('contract', False) %}
      {{ get_assert_columns_equivalent(sql) }}
      {{ get_columns_spec_ddl() }}
      {%- set sql = get_select_subquery(sql) %}
    {% endif %}
  as (
    {{ sql }}
  );
{%- endmacro %}

BigQuery create_table_as: https://github.com/dbt-labs/dbt-bigquery/blob/main/dbt/include/bigquery/macros/adapters.sql#L57

We won't need to do this for view materializations as they don't support constraints.