Motivation
Many tools support an ini file for configuration (see #185, #219, #221, and #280). Although it's always preferable to use pyproject.toml, ideally we use whatever configuration approach is already being used and work with it.

This means each tool needs both managed pyproject.toml sections and managed sections for other files too. Some tools have bespoke files (e.g. ruff.toml). So we need some abstraction for how to describe:

• What the config section should look like for different files (note, this could involve a fair bit of (non-accidental?) duplcaition)
• A heuristic for detecting which file to use for configuration (but what do we do when multiple files have configuration simultaneously already, or if it is standard practice for this tool to be configured in multiple files (but this doesn't happen yet so can we just say YAGNI?)

Summary of desired enhancement
Provide ini file support, especially for setup.cfg and appropriately modify the config file management abstractions for tools.