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.

Comienza localizando los puntos de entrada CdkStepper y CdkStep en el repositorio de componentes de Angular e inspecciona cómo se fija actualmente el tipo de paso. Confirma el comportamiento deseado con un MyStep o OnboardingStepComponent especializado y, después, añade cobertura que demuestre que CdkStepper admite un paso extendido con specialLabel.

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