[PR] Add cornerRadius to ApplePayButtonView (issues#127)

[AymenFarrah]

Customize Apple Pay Button (cornerRadius)

The goal of this PR is to enable customization of ApplePayButton cornerRadius

By default the value is set to 4.0 (https://developer.apple.com/documentation/passkit/pkpaymentbutton/2999416-cornerradius) but we are able to override this value.

So in this PR we add a new attribute cornerRadius to custom the Apple Pay Button cornerRadius

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[JlUgia]

Hi @AymenFarrah, thank you for your contribution. The Google Pay button announced the ability to adjust the corner radius at Google I/O this year, and hence it'd be valuable to add consistent support for the two platforms. I suggest we coalesce both APIs and ensure they are cohesive.

I'm currently putting together the Android version. Let me finish that and circle back with you to make sure both PRs are aligned.

[JlUgia]

Good day @AymenFarrah. A similar change was included in #243. I'd still like to give you credit for your proposal. In addition, this proposal includes an iOS version check before setting the cornerRadius on the iOS end. If you can pull from main and update this PR, I can help review and merge it. Thank you.