Add #[schema(skip)] option for enum variants

[notNotDaniel]

Adds a new #[schema(skip)] option which can be used to filter out enum variants before deciding if the set of variants is mixed or composed only of Unit values. We need this to support our use case, where we have enums with a set of known values, but also want to capture a single Unknown(String) value using #[serde(untagged)], but need the resulting openApi documentation to include just a simple list of enum values.

I had hoped to reuse the existing ignore feature, however since the filtering needs to happen during the schema generation (but not rust code generation), it wasn't clear how the expression included with ignore could be evaluated, and the simple skip option matches nicely with the serde skip option.