[ViewModelType] Provide a clear contract between inputs/outputs desired in a view model

HelloWorldViewModel.swift

final class HelloWorldViewModel: ViewModelType {
  let input: Input
  let output: Output
  
  struct Input {
    let name: Anyobserver<String>
  }
  
  struct Output {
    let greeting: Driver<String>
  }
  
  private let nameSubject = ReplaySubject<String>.create(bufferSize: 1)
  
  init() {
    let greeting = nameSubject
    	.map { name in
              return "Hello \(name)"
        }
    	.asDriver(onErrorJustReturn: "Hello Doe")
    
    self.output = Output(greeting: greeting)
    self.input = Input(nameSubject.asObserver())
  }
}

ViewModelType.swift

protocol ViewModelType {
    associatedtype Input
    associatedtype Output

    var input: Input { get }
    var output: Output { get }
}