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

test/controllers/plaid_items_controller_test.rb

@@ -8,7 +8,7 @@ class PlaidItemsControllerTest < ActionDispatch::IntegrationTest
 
   test "create" do
     @plaid_provider = mock
-    Provider::Registry.expects(:get_provider).with(:plaid_us).returns(@plaid_provider)
+    Provider::Registry.expects(:plaid_provider_for_region).with("us").returns(@plaid_provider)
 
     public_token = "public-sandbox-1234"