feat: add case conversion strategies to include/exclude keys

[moldhouse]

This PR introduces two new strategies for case conversion, which allow users to either specify a list of keys that should be converted or exclude a list of keys from conversion. This solves issue #625 that it is currently not possible to exclude some keys from case conversion.

[epage]

I'm marking this as a Draft because we don't look at PRs until a problem is understood and and a solution is agreed upon, per our contributing guide