Add swiss post address validation

Primsi thank you for your contribution! I'm currently working on other projects. I'm going to have a look into this and provide feedback soon. I can't provide an ETA tough.

No worries, Primsi is working with this on me and I'll review it.

We've developed this before for a client project and are open-sourcing it now, basically.

Ok, great! Thank you.

One thing a bit off-topic: Could we extend this to support something similar to https://www.drupal.org/project/address_autocomplete_gmaps ?

The API we use does have autocomplete endpoints too, so yes. There is a street endpoint that will return suggestions for a full address.

That's not in scope of what we're doing here, but a follow-up could build on that.

The module does implement that by providing its own widget. I do wonder if that would be a more sensible approach to the magic we try to do here right now, would also give us a place to put options, like an autocomplete yes/no checkbox.

@berdir sure, a follow-up makes more sense.

Exactly, that's why I suggested it here. But the autocomplete approach also comes with a lot of disadvantages, if solved poorly and can result in a worse UX than before. address_autocomplete_gmaps is actually a good example. It breaks the address form's UX quite a bit. For example editing the values again is quite cumbersome.

There is a typo in the service's name: `commerce_swiss_post.adress_verification` It should be: `commerce_swiss_post.address_verification`.

I think we want the same behavior there, auto-correct and also the resubmit part. Otherwise looks good I think.

One more question; Do we really need sessions for this? Can't the re-submission functionality be achieved with form state? It feels weird to store all addresses in sessions...

This was tested and does what it's meant to. There are some possible additional feature and improvements, like configurable action on returned changes (some are a bit strange) as well as supporting autocomplete in some form as well, but we can look into that if someone is interested in sponsoring that.