feat: allow `OptionalAdapter` to accept more values

Is it possible to implement something like this:

```swift
OptionalAdapter(maybeString, maybeInt) { string, int in
  // ...
}
```

or

```swift
OptionalAdapter(maybeString, maybeInt) { tuple in
  // tuple.0, tuple.1
}
```

Thank you for development of Turbocharger! (and Engine)

Inspiration:
https://github.com/swiftlang/swift-evolution/blob/main/proposals/0393-parameter-packs.md