CSS4 box-shadow variable for toolbar-background

# Feature Request
Please add a css4 box-shadow variable for ion-header ion-toolbar:last-child or ion-footer ion-toolbar:first-child.

**Ionic version:**
[x] **4.x**

**Describe the Feature Request**
Ionic 4 has no satisfying option to add a box-shadow to the header in iOS mode. Adding a box-shadow to the ion-toolbar or ion-header does not work, because it leaves an ugly navigation animation if the header is not the same size on the parent / child pages.


**Describe Preferred Solution**
CSS4 box-shadow variable for ion-header ion-toolbar:last-child or ion-footer ion-toolbar:first-child. This was the fix for adding a good looking box-shadow to toolbars in Ionic 3 and is prevented with the new shadow DOM.

**Additional Context**
The solution given in https://github.com/ionic-team/ionic/issues/16895 does not fix the problem because then the navigation animation looks bad.

Start by locating the iOS-mode styles for ion-header, ion-footer, and ion-toolbar, then review how existing toolbar CSS variables are defined. Verify the change against the reported navigation animation case and confirm that the requested box-shadow variable can style the relevant first or last toolbar without altering layout.

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