feat(material/stepper): add a styling token for header icon shape

[sdedieu]

Fixes: #33962

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[sdedieu]

Thanks for the approval @crisbeto 🙏
Regarding this, I was wondering if you have a rule or guidelines about what can be customizable and what will never be ?
To give you some context, I work at Criteo and our design system for Angular is implemented on top of Angular Material. For some components we still have to perform bad CSS overrides to achieve what our UI team is requesting. The more and more styling tokens exists, the more and more we're able to do it the right way.
I would be more than motivated to contribute as much as possible to the angular/components codebase to add as many styling tokens as we need. Just wondering if you have somewhere a document where I can see what will never be possible and share it to my UI team 🙂

[crisbeto]

We don't have specific guidelines on this. Initially we set up the tokens based on what's in the Material Design spec, but since then we've been adding tokens wherever we think it makes sense.