[w3c/webpayments] Payment Method Manifest spec - should enable country specific app discovery (#224)

**Problem:** 
The https://w3c.github.io/webpayments/proposals/Payment-Manifest-Proposal.html doesn't appear to enable User Agents to discover which app to discover and instantiate for payment handlers that have country specific versions. (Samsung Pay, for instance, has a different .apk for different markets: US, KR, etc.)
**Input/Analysis:**
1. There are different country/market indicators that could come from different players (device, browser/userAgent, merchant), which might be used to help determine which app to instantiate - but there are problems with each of them. 
2. Different globe travelers have varying interests in using different country versions of an app. 
3. A common way to express countries would be needed, and to embed them in the payment manifest file.

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https://github.com/w3c/webpayments/issues/224