Add method to parse bank transactions as JSON

This implements parsing of the Account Information Service format as
defined in the Berlin Group Group NextGenPSD2 XS2A Framework, which
is widely used across various European banks.

This is a first step to replace the current bank import features with
a standardized JSON interface.

app/models/bank_account.rb

@@ -24,4 +24,8 @@ class BankAccount < ApplicationRecord
     end
     count
   end
+
+  def last_transaction_date
+    bank_transactions.order(date: :desc).first&.date
+  end
 end