Make *KeyReader generic once accessible macro starts to support generic E and O

[bmarinovic]

Currently, accessible zio-macro does not support generic E and O types in Service: zio-archive/zio-macros#60

Once this is fixed, KeyReaders should be abstracted, being polymorphic in output type:

@accessible(">")
trait KeyReader[A] {
  val serviceAccountKeyReader: KeyReader.Service[Any, A]
}



object KeyReader {
  trait Service[R, A] {
    def readKey(path: String): ZIO[R, KeyError, A]
  }
}