 ForcePKCE  option make orther grant type error

The origin validate ForcePKCE in server.ValidationTokenRequest func:
```
if s.Config.ForcePKCE && codeVer == "" {
  return "", nil, errors.ErrInvalidRequest
}
```
 Should add filter by grant type authorization code?

```
if gt == oauth2.AuthorizationCode && s.Config.ForcePKCE && codeVer == "" {
  return "", nil, errors.ErrInvalidRequest
}
```