[CT-2220] [Snowflake] Add model-level constraints to get_columns_spec_ddl

[MichelleArk]

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

Snowflake supports not_null, primary_key, foreign_key, and unique constraints:

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

constraints specified at the model-level should be rendered to the column spec ddl after column-level constraints are rendered: https://github.com/dbt-labs/dbt-snowflake/blob/main/dbt/include/snowflake/macros/utils/get_columns_spec_ddl.sql#L10-L11.

Constraint docs: https://docs.snowflake.com/en/sql-reference/sql/create-table-constraint