Add filter to not_null_proportion test

[j-svensmark]

Describe the feature

I want to add a filter as an optional argument to the not_null_proportion test.
This is make it possible to use this test in e.g. a partitioned table in BQ with a required filter over a given column.

I made possible implementation in main...j-svensmark:dbt-utils:patch-1 (untested though), though I saw your PR policy doesn't allow PRs without an issue, hence this issue.

[j-svensmark]

Ah, just found that the built in where functionality in dbt tests (https://docs.getdbt.com/reference/resource-configs/where) covers this use case, so closing this issue