On the Blanket Order PDF (as well as other purchase agreements), the
vendor isn't specified on the report while it is a required and
significative field on the purchase agreement.
Added it like in the purchase order PDF.

Also added in the Custom Description field into the PDF as it may
contain useful information as well.

Task-3358385

closes odoo/odoo#125014

Signed-off-by: Arnold Moyaux (arm) <arm@odoo.com>