feat(Sidenav): Add additional palette definitions on a component basis

[intellix]

Feature Description

We would like to theme various components without affecting another unrelated component. One particular example, is to change the background colour of the sidenav without changing the dialog background. Currently the sidenav/drawer palette variable uses the same background variable used for dialog.

Sidenav is one example, but I've seen other components re-using variables from other components:

https://github.com/angular/components/blob/34a43f62233790d083f2c0a041fc6740640584cd/src/material/sidenav/_sidenav-theme.scss#L14-L17

These two in particular are obvious to me:

$drawer-background-color: mat-color($background, drawer); 
$drawer-push-background-color: mat-color($background, drawer); 

And in the palette it's just an extra definition:

$mat-dark-theme-background: (
  ...
  dialog:     map-get($mat-grey, 800),
  drawer:     map-get($mat-grey, 800),
);

Use Case

Our site is built dynamically using a CMS and we would like to offer the ability to change the background colour of the drawer without affecting the background of modals.

I've been meaning to write this issue for years now but keep forgetting... so here it is

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