[IMP] purchase: purchase report improvement

Modify purchase report in order to make it consistent with
sales report.
For the RFQ it will show the Order Date and for the Purchase order
menu it will display the Confirmation Date.

Also rewrite a bit the SQL view in order to compute every purchase
order in the company currency.

Technicaly rewrite the SQL alias in order to have a report more
readable in the future.

Task ID : 1857130.

closes odoo/odoo#28248

Signed-off-by: Arnold Moyaux <amoyaux@users.noreply.github.com>