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/models/plaid_item/importer_test.rb

@@ -0,0 +1,23 @@
+require "test_helper"
+require "ostruct"
+
+class PlaidItem::ImporterTest < ActiveSupport::TestCase
+  setup do
+    @mock_provider = PlaidMock.new
+    @plaid_item = plaid_items(:one)
+    @importer = PlaidItem::Importer.new(@plaid_item, plaid_provider: @mock_provider)
+  end
+
+  test "imports item metadata" do
+    PlaidAccount::Importer.any_instance.expects(:import).times(PlaidMock::ACCOUNTS.count)
+
+    PlaidItem::Importer.new(@plaid_item, plaid_provider: @mock_provider).import
+
+    assert_equal PlaidMock::ITEM.institution_id, @plaid_item.institution_id
+    assert_equal PlaidMock::ITEM.available_products, @plaid_item.available_products
+    assert_equal PlaidMock::ITEM.billed_products, @plaid_item.billed_products
+
+    assert_equal PlaidMock::ITEM.item_id, @plaid_item.raw_payload["item_id"]
+    assert_equal PlaidMock::INSTITUTION.institution_id, @plaid_item.raw_institution_payload["institution_id"]
+  end
+end