Column Mock improvements

[mcnick]

What problem does this PR solve

The PR contains multiple small improvements to the usability of Column Mocks

What changed

• made column mock nullable by default
• array inner types use other column mocks
• added BigQuery column mocks

Checklist

• I added tests for new functionality if applicable
• I added an entry for changes to the unreleased changes of CHANGELOG.md
• I added documentation for new functionality if applicable
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• New and existing unit tests pass locally with my changes

[Somtom]

Only some minor things. Thanks for tackling that.