// LoadPEMPrivKeyPKCS8 reads a PEM-encoded RSA private key in PKCS8 format from
// the given reader.
//
// [x509.ParsePKCS8PrivateKey] can return multiple key types and this API does
// not discern between them. Callers in need of the underlying value must
// obtain it via type assertion:
//
//  key, err := sign.LoadPEMPrivKeyPKCS8(r)
//  if err != nil { /* ... */ }
//
//  switch key.(type) {
//  case *rsa.PrivateKey:
//      // ...
//  case *ecdsa.PrivateKey:
//      // ...
//  case ed25519.PrivateKey:
//      // ...
//  default:
//      panic("unrecognized private key type")
//  }
//
// See aforementioned API docs for a full list of possible key types.
//
// If calling code can opaquely handle the returned key as a [crypto.Signer],
// use [LoadPEMPrivKeyPKCS8AsSigner] instead.
func LoadPEMPrivKeyPKCS8(reader io.Reader) (interface{}, error)

// LoadPEMPrivKeyPKCS8AsSigner wraps [LoadPEMPrivKeyPKCS8] to expect a
// [crypto.Signer].
func LoadPEMPrivKeyPKCS8AsSigner(reader io.Reader) (crypto.Signer, error)