[IMP] account: compute foreign amount on bank transactions

With this commit, we improve the usability of foreign bank transactions.
When capturing a new bank transaction (account.bank.statement.line)
- when capturing a new transaction we compute the amount currency based on the transaction date, amount in journal/company currency, and foreign currency
- when the foreign currency is removed, the amount_currency is cleared

Task-3338560
Task-3338532

closes odoo/odoo#122499

Related: odoo/enterprise#41503
Signed-off-by: Laurent Smet <las@odoo.com>