Plaid sync domain improvements (#2267)

Breaks our Plaid sync process out into more manageable classes. Notably, this moves the sync process to a distinct, 2-step flow:

1. Import stage - we first make API calls and import Plaid data to "mirror" tables
2. Processing stage - read the raw data, apply business rules, build internal domain models and sync balances

This provides several benefits:

- Plaid syncs can now be "replayed" without fetching API data again
- Mirror tables provide better audit and debugging capabilities
- Eliminates the "all or nothing" sync behavior that is currently in place, which is brittle

app/models/provider/registry.rb

@@ -18,6 +18,10 @@ class Provider::Registry
       raise Error.new("Provider '#{name}' not found in registry")
     end
 
+    def plaid_provider_for_region(region)
+      region.to_sym == :us ? plaid_us : plaid_eu
+    end
+
     private
       def stripe
         secret_key = ENV["STRIPE_SECRET_KEY"]