Fix Issue #235

[dickermoshe]

Bug:

Fixes issue #235
The current implementation of the ApplePayButton only sets the payment items on create.
Changing the payment items after the button is created doesn't change the callback from onPressed.

Changes:

• _UiKitApplePayButton sets the callback on each build.
• The example has been updated with quantity, this will show why this PR is needed.

Before:

Changing quantity didn't change the price

Simulator.Screen.Recording.-.iPhone.SE.3rd.generation.-.2023-12-26.at.14.04.54.mp4

After:

Price changes due to quantity

Simulator.Screen.Recording.-.iPhone.SE.3rd.generation.-.2023-12-26.at.14.03.55.mp4

[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.

[dickermoshe]

@JlUgia
Could you please take a look at this when you get a chance, it's a pretty significant bug

[JlUgia]

Thank you for your contribution @dickermoshe.
I left a couple of comments for you to take a look.

[JlUgia]

We'll also need to merge the conflicts before submitting this.
Let me know if you need any help with that.

[dickermoshe]

Rebased on main, and removed changes from the example app.
Also changed the fix to just use a stateful widget.
Thanks

[JlUgia]

Thank you @dickermoshe.
Merging.