Implement average aggregation for decimal in SQLite

[ranma42]

Contributes to #19635

• I've read the guidelines for contributing and seen the walkthrough
• I've posted a comment on an issue with a detailed description of how I am planning to contribute and got approval from a member of the team
• The code builds and tests pass locally (also verified by our automated build checks)
• Commit messages follow this format:

        Summary of the changes
        - Detail 1
        - Detail 2

        Fixes #bugnumber

(well, "Contributes" instead of "Fixes")

• Tests for the changes have been added (for bug fixes / features)
• Code follows the same patterns and style as existing code in this repo

[ranma42]

I also posted #33721. This PR is slightly simpler as it does not affect the nullability processing, but I left it as a draft as I still have some (similar) doubts around both these changesets. I'll update both PRs later on 😇

[roji]

@ranma42 I'd suggest submitting a single PR for the multiple aggregate translations; things are similar enough that it makes sense to just have a single PR I think.