Add optional support for enabling/disabling tests that you want considered for constraints

[josephlozano]

Use variables to support option to enable and disable constraints applying to models; this follows the existing pattern applied to sources. Happy to start the PR if this is the approach to take and will be a benefit to the package.

dbt_project.yml

# Global variables
vars:
  # The package can be temporarily disabled using this variable
  dbt_constraints_enabled: true

  # You can be specific on which constraints are enabled for models
  dbt_constraints_pk_enabled: true
  dbt_constraints_uk_enabled: true
  dbt_constraints_fk_enabled: true
  dbt_constraints_nn_enabled: true

  # The package can also add constraints on sources if you have sufficient privileges
  dbt_constraints_sources_enabled: false

  # You can also be specific on which constraints are enabled for sources
  # You must also enable dbt_constraints_sources_enabled above
  dbt_constraints_sources_pk_enabled: true
  dbt_constraints_sources_uk_enabled: true
  dbt_constraints_sources_fk_enabled: true
  dbt_constraints_sources_nn_enabled: true

macros/create_constraints.sql

{%- macro create_constraints(
...
        {%- if 'not_null' in constraint_types and var('dbt_constraints_nn_enabled', false) -%}
            {%- do dbt_constraints.create_constraints_by_type(['not_null'], quote_columns, lookup_cache) -%}
        {%- endif -%}
...