[layout] nb-sidebar with fixed and nb-header with subheader

### Issue type

**I'm submitting a ...**

* [ ] bug report
* [X] feature request

### Issue description

I am using `nb-layout`, `nb-layout-header` and `nb-sidebar` (also an `nb-layout-column` with `router-outlet`)
The sidebar has `containerFixed=false`

I want the sideBar to overlap mi header in a fixed way (not pushing the header and column to the side)
That's why my header is a `subheader`

**Current behavior:**
When I add the `fixed=true` to the side bar it gets on top of the content but leaves the header space as if it didn't had the `containerFixed` set to `false`

When I do the inspection with chrome's devTools I find this style setting a `top:--header-height` when I disable it I get the expected result
```
nb-layout .layout .layout-container nb-sidebar.fixed,
nb-layout .layout .layout-container nb-sidebar .main-container-fixed {
top: var(--header-height);
}
```
Then I set the `containerFixed=true` and also got the same result and as the header is not `fixed` it leaves an empty space were you can see the content below moving as you scroll.

**Expected behavior:**
I want the sideBar to overlap mi header in a fixed way (not pushing the header and column to the side)
And be able to fix the header too and that the sidebar is on top of the header. But don't think it will get affected by this bug fix.

I believe is a styles bug and I will be happy to help you guys fix this.

**Steps to reproduce**
Are simple
1. add a `nb-layout`, `nb-layout-header`, `nb-sidebar` and `nb-layout-column` with `router-outlet`
2. In the nb-layout-header set `subheader`
3. In the nb-sidebar set `fixed=true`
4. Write something to show on header, sidebar and content

Hope this is understandable.

EDIT:
Also removed the `subheader` form the `nb-header` and also getting the awful transparent space when I scroll. So I believe that the `fixed`
is the problem or maybe is not design to do what I need and this is not a bug but a feature request.

So what's the expected behavior for the `nb-sidebar` `fixed=true`?
Overlapping the content, but not the header I suppose

**Then this is a feature request**
So the feature is that when an `nb-header` is `subheader` and the `nb-sidebar` is `fixed` the `top=0`,