Add referenced schema to foreign key constraints

[jtpalmer]

Description

Add support for foreign key constraints that reference a schema that is different from the one that they belong to. Also refactor some tests.

Motivation and Context

It's possible for a foreign key constraint to reference a table in a different schema so the database model should support that.

See https://app.asana.com/0/15230011293693/1163079712082841

Tests performed

Added new tests.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project as found in the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.