mat-stepper: Allow custom CSS classes for mat-step-header

#### Please describe the feature you would like to request.
Please add API to add custom css classes to `mat-step-header`.

#### What is the use-case or motivation for this proposal?
We want to set custom cursor for disabled steps in linear stepper.

#### Is there anything else we should know?
Disabled steps have the same hover and click animations, it would be nice to be able to customize them. But I guess there is other issue for that. And custom css classes would also help with this.

Start at the mat-step-header entry point and inspect how the stepper exposes header configuration and styling. Determine how custom CSS classes should be supplied and applied, then verify that the classes are present on step headers, including disabled steps, with focused component tests.

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