feat(stepper): Make `CdkStepper` generic to support extended CdkStep

#### Feature Description

Currently, when I want to use `CdkStepper` with a custom class for a step (extension of a class `CdkStep`, e.g. `MyStep extends CdkStep`), I am not able to as `CdkStepper` is not generic and is fixed to `CdkStep` class for a step. I would like to make `CdkStepper` generic, so I can use `CdkStepper`.

#### Use Case

Let's say I have a class`OnboardingStepComponent extends CdkStep` which has the property `specialLabel`. I want to use the CdkStepper with this specialized step component.

Let me know if this sounds reasonable, if so, I can work on this.

Start by locating the CdkStepper and CdkStep entry points in the Angular components repository and inspect how the step type is currently fixed. Confirm the desired behavior with a specialized MyStep or OnboardingStepComponent, then add coverage showing that CdkStepper supports an extended step with specialLabel.

Written by the indexing model from the issue text.