Add support for country-specific fixtures in trigger cmd

[edd-stripe]

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cc @stripe/developer-products

Summary

Adds some additional "variants" to triggers that are country-specific.

Primary motivation is to provide better CLI support for users whose account contexts don't allow them to use the values hardcoded in the existing fixtures (for e.g. Issuing users in the EU are not permitted to issue cards to cardholders in the US) - and to do so as first-class support, instead of requiring those users to bring their own fixtures or provide overly complex override flags.

In other words, this should be considered a native replacement for frequently-used patterns of override flags or scenarios where users often provide similar custom fixtures.

Usage examples

$ stripe trigger issuing_authorization.request # default, US
$ stripe trigger issuing_authorization.request.gb # new
$ stripe trigger issuing_authorization.request.eu # new

Examples of previous usage to achieve the same effect:

$ stripe trigger issuing_authorization.request \
  --override cardholder:billing.address.country=GB \
  --override cardholder:phone_number=+440000009900 \
  --override card:currency=gbp \
  --add authorization_request:held_currency=gbp

$ stripe trigger issuing_authorization.request \
  --override cardholder:billing.address.country=ES \
  --override cardholder:phone_number=+34000000990 \
  --override card:currency=eur \
  --add authorization_request:held_currency=eur

$ stripe fixtures some_json_file_provided_by_the_user.json

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