[final][hold] feature: watch example

[aboedo]

added a sample watch app to the swift example.

it only has one screen, where you see your status and make a purchase.

It's on HOLD because it seems like on the current watchOS 6.2 beta, the receipt comes back empty. I'm hoping that the next seed will fix this

[aboedo]

updated this example:

• instead of being a new target in the existing example, it's a new example under the examples folder.
• the new example is an independent watch app with no corresponding iOS app.

[aboedo]

@vegaro bump