Add paginate_rows() method

[jwodder]

This PR adds a paginate_rows() method to the extension object that behaves like paginate(), except its items are sqlalchemy.Row instances instead of whatever was the in the first column of the select(). This makes it possible to paginate compound selects without having to fall back to the legacy Query API.

• Fixes Support select(Model.column1, Model.column2) in pagination  #1168.

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.