[TOASTR]: Add ability to position toast in top center

### Issue type

**I'm submitting a**

* [ ] bug report
* [x] feature request

### Issue description

**Current behavior:**
Currently, NbGlobalPosition allows only to use TOP_RIGHT, TOP_LEFT, BOTTOM_RIGHT, BOTTOM_LEFT positions.

**Expected behavior:**
NbGlobalPosition should allow to specify TOP_CENTER, BOTTOM_CENTER positions.

### Other information:
Currently, it is hardly possible to extend ToastrModule due to the fact, that NbPositionHelper, NbPositionBuilderService and NbToastrContainerRegistry classes depend on the concrete enum NbGlobalLogicalPosition.