feat: fixed toolbars with iOS large header

# Feature Request

**Ionic version:**

[x] **4.x**

**Describe the Feature Request**

Just mentioned this in the native GitHub App: The Searchbar and other Toolbars seems to have the option to be fixed to the large header and don't collapse on scroll (with Ionic the Toolbars are collapsed currently).

I think this is different from `slot="fixed"` as these toolbars are within the header and also change the background color with it.

**Describe Preferred Solution**

Maybe a **collapse** attribute for `ion-toolbar` which indicates the wanted behaviour.

Start by reviewing the Ionic 4.x ion-toolbar and header behavior, then compare it with the native GitHub App example shown in the issue. Define how a toolbar inside the header should remain fixed to the large iOS header and transition its background while scrolling; done means it no longer collapses like the current toolbars.

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