Remove unnecessary include for integration tests.
Ensure we always use instances of the adapter specific column class

- Create a consistent API across adapters for building new columns
- Use it for custom properties so we don't get `UndefinedMethodError`s
  in stuff I'm implementing elsewhere.
[ci skip] Make last note show up in postgresql guide.
Merge pull request #15403 from JuanitoFatas/doc/pg-guide-note

[ci skip] Make last note show up in postgresql guide.
Merge pull request #15397 from sgrif/sg-column-class

Ensure we always use instances of the adapter specific column class
Merge pull request #15061 from tgxworld/remove_unnecessary_include

Remove unnecessary include for integration tests.
Treat invalid uuid as nil
Test for _before_type_cast in case of invalid uuid.
Pull RFC_4122 Regex as constant.