ConfigReader methods that take a transform closure

It might be useful to have a transform variant:

```swift
let foo = config.string(forKey: "foo", as: Foo.self, default: .bar) { rawString in
// turn string into a Foo
}
```

Start by locating the ConfigReader methods that read typed values and accept defaults, then compare their existing overloads with the proposed transform-closure form. Done means the relevant methods support the shown string-to-Foo conversion while preserving default handling and the existing API behavior.

Written by the indexing model from the issue text.