[CT-2222] [Redshift] Add model-level constraints to get_columns_spec_ddl

[MichelleArk]

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

Redshift supports not_null, primary_key, foreigh_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-redshift/blob/main/dbt/include/redshift/macros/utils/get_columns_spec_ddl.sql#L35

Constraint docs: https://docs.aws.amazon.com/redshift/latest/dg/r_CREATE_TABLE_NEW.html