fix: correct overriding border radius

[lukewalczak]

Fixes: #3924

Summary

Allow customizing borderRadius value in Card.Cover.

Test plan

Adding new example and unit test case.

[callstack-bot]

Hey @lukewalczak, thank you for your pull request 🤗. The documentation from this branch can be viewed here.

[github-actions]

The mobile version of example app from this branch is ready! You can see it here.

[meatnordrink]

I am noticing what might be an unintended side-effect to this improvement; now, if you pass in rounding to a single corner (as we were already doing in a number of places), that value becomes the default for all corners of the <Card.cover>. For example, this is what we're seeing, after upgrading and pulling in this work:

  cardCover: {
    borderTopRightRadius: 8, /* All four borders have 8px rounding */
    }

It's possible that this is the intended behavior, I suppose, as I can see an argument for it. And the work around is simple, just passing in values for each corner, e.g.

  cardCover: {
    borderTopRightRadius: 8,
    borderTopLeftRadius: 0,
    borderBottomRightRadius: 0,
    borderBottomLeftRadius: 0,
  },

If it was intentional, no worries; just figured I'd check that it was. If it wasn't, let me know and I could look at submitting a fix.