feat: make cleanCargoToml customizable

[benedikt-schaber]

Motivation

Currently cleanCargoToml hardcodes which paths to remove. This PR allows users to pass a pathsModifier function to cleanCargoToml to make it customizable. Further, this PR takes it from a "what to remove" to a "what to keep" approach, which seems more scalable to me. However, this means that it is a breaking change since unknown fields are now removed by default instead of kept by default.

This was motivated by me using different cargo-tools that relied in some form or another on fields filtered by cleanCargoToml (e.g. cargo-leptos uses workspace.metadata.leptos), which can then be easily adjusted here instead of having to roll my own cleanCargoToml. But could also make it easier to implement #288, I think. I could work on that if this seems like a good direction.

There are lib functions which mostly do what my helper functions do, however, these do not handle lists, while my functions do (by iterating over the children and applying the path filtering there too).

Checklist

• added tests to verify new behavior
• added an example template or updated an existing one
• updated docs/API.md (or general documentation) with changes
• updated CHANGELOG.md