No option to exclude keys from case conversion

Currently, it is not possible to specify a list of keys that should be excluded from case conversion.

In my case, this makes it hard to build a config from an Environment and a File, where some identifiers in the file are kebab-case, while other parameters are snake_case.

For example, I want a kebab-case key and a snake_case configuration:

# TOML configuration
[foo.kebab-case-key]
snake_case_configuration = "bar"

# Environment configuration
FOO__KEBAB_CASE_KEY__SNAKE_CASE_CONFIGURATION=bar

While there is a method convert_case, it is applied to all collected keys. For this example, a possible solution would be something like:

Environment::default().convert_case_exclude_keys(Case::Kebab, ["snake_case_configuration"]);

Start by reading Environment::with_convert_case and its documentation to understand how case conversion is applied. Add support for excluding named keys while converting other keys, and verify that the kebab-case and snake_case configuration example can be represented as described.