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

[lukasmatta]

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<MyStep>.

Use Case

Let's say I have a classOnboardingStepComponent 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.

[abdulkareemnalband]

We also had faced same issue, we worked around by defining new getter like

class BespokeStepper extends CdkStepper{
    get selectedStep() {
        return this.selected as MyStep;
    }
}

And using that in template

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