feat(nulltest): add not_null option to unique_combination_of_columns

[halfos]

Problem

We perform unique and not null tests on all primary key columns. Previously, not_null data tests had to be specified repeatedly for the same columns that are also listed in the unique_combination_of_columns test.

Solution

The proposed solution adds an optional for loop that performs not null tests on each column in the unique_combination_of_columns test. Gives neater code. The default parameter is false, so it should not be a breaking change.

Describe the feature

A neat way to enforce primary keys is to have an option to not only use the "unique_combination_of_columns" test for unique tests of composite keys, but also for enforcing the not_null constraint for each of the underlying columns. This way developers have the option of not having to individually repeat single not_null tests for each of the columns listed in the "unique_combination_of_columns" test. This results in better readability, and hence better code quality.

Describe alternatives you've considered

Have considered writing a separate macro which combines these two types of tests, but I think it seemed more useful to have the not_null option inside the unique_combination_of_columns macro.

Additional context

Primary keys are not enforced on Snowflake, but unique_combination_of_columns and not_null can be used as a way around to enforce it.

Who will this benefit?

Enforcing primary keys in all databases.

Are you interested in contributing this feature?

Yes, already have a pull request ready (simple for loop).

[halfos]

Realized now that putting them together in one test may make it difficult to see which part of the test failed

[halfos]

A major flaw with this suggestion as it is coded right now is that when the test fails, it does not say whether it is the unique combination or not_null part that fails.

[halfos]

Might be better to concatenate composite keys into a single primary key column, and use a simple unique and not_null test separately as hinted in the docs: https://docs.getdbt.com/reference/resource-properties/constraints