refactor: extract permitted_attribute_names from permitted_attributes

[milgner]

In one of our controllers where we use delegated types and nested attributes, we needed to check the permitted attributes separately for the delegated type. This splits the method permitted_attributes in two, separating finding the attribute names and looking them up in the params, so that the attribute names can be retrieved individually.

[Burgestrand]

Hi! Sorry about you not receiving a reply at all. 2022 was hectic, so while we did have a brief look the reply slipped our minds.

I'm generally positive about this. I'm slightly worried about expanding our public API, especially when the namespace is shared between pundit and whatever application is using this.

I'm pondering if we either prefix this method pundit_permitted_attribute_names, or if we expose this only through the Pundit::Context. I need to simmer that for a bit.