Allow NbPasswordAuthStrategy.logout() without HTTP request

### Issue type

**I'm submitting a ...** (check one with "x")

* [ ] bug report
* [x] feature request

### Issue description

**Current behavior:**
Using `NbPasswordAuthStrategy.logout()` there's always an HTTP request for the `logout` endpoint. But when using stateless JWT, the backend API will not have a `logout` endpoint.

**Expected behavior:**
Allow the configuration of `logout` (and other functions) without making an HTTP request. Some functions could require HTTP request (e.g. `login`), but others don't.

**Steps to reproduce:**
We need a backend API that doesn't have a `logout` endpoint, but has a `login` endpoint that provides a stateless JWT.