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/plaid_item/provided.rb

@@ -0,0 +1,7 @@
+module PlaidItem::Provided
+  extend ActiveSupport::Concern
+
+  def plaid_provider
+    @plaid_provider ||= Provider::Registry.plaid_provider_for_region(self.plaid_region)
+  end
+end