Create StripeCountrySpec model and tests

# Problem

We want a StripeCountrySpec that contains most data from this Stripe API: https://stripe.com/docs/api#country_spec_object

We can use Stripe's `id` as our `id`. For now, there will not be an API endpoint in our application to create/retrieve/update/delete these.

Instead, these records will be created and then will be updated regularly with a cron-like job to find the latest information for displaying the form fields that a user might possibly need for creating their Stripe account and verifying their identity, either as an individual or as a company representative.

The `StripeCountrySpec` will belong to a `StripeConnectAccount`, and we'll set the `stripe_country_spec_id` on the `StripeConnectAccount` in order to render form details out to the user.

We likely need to use embedded fields here in order to handle the deeply nested hash for the verification fields https://hexdocs.pm/ecto/Ecto.Schema.html#embeds_one/3

We probably want to simplify this to a combination of `IndividualMinimumVerificationFields`, `IndividualAdditionalVerificationFields`, `CompanyMinimumVerificationFields`, and `CompanyAdditionalVerificationFields` schemas, where the key/value pair is the field name (e.g. `legal_entity.address.city`) and a boolean.

For now, we can probably ignore implementing anything around `supported_bank_account_currencies`, `supported_payment_currencies`, and `supported_payment_methods`. We should track the `default_currency`, though.