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_account/importer.rb

@@ -0,0 +1,34 @@
+class PlaidAccount::Importer
+  def initialize(plaid_account, account_snapshot:)
+    @plaid_account = plaid_account
+    @account_snapshot = account_snapshot
+  end
+
+  def import
+    PlaidAccount.transaction do
+      import_account_info
+      import_transactions if account_snapshot.transactions_data.present?
+      import_investments if account_snapshot.investments_data.present?
+      import_liabilities if account_snapshot.liabilities_data.present?
+    end
+  end
+
+  private
+    attr_reader :plaid_account, :account_snapshot
+
+    def import_account_info
+      plaid_account.upsert_plaid_snapshot!(account_snapshot.account_data)
+    end
+
+    def import_transactions
+      plaid_account.upsert_plaid_transactions_snapshot!(account_snapshot.transactions_data)
+    end
+
+    def import_investments
+      plaid_account.upsert_plaid_investments_snapshot!(account_snapshot.investments_data)
+    end
+
+    def import_liabilities
+      plaid_account.upsert_plaid_liabilities_snapshot!(account_snapshot.liabilities_data)
+    end
+end