Commit 044c5c2c81 removed the context key
`variant` of the bom structure report. This implies to always print
every variant in case a BoM has no product_id.
This commit fix this issue to print only the variant selected in the
report. It also adds a new button to print the report with all variants
in case for some customers this behavior was "not a bug but a feature".
Task : 2504524
closes odoo/odoo#70444
X-original-commit: 8da349e785b5b461ea19874df9686f8cd26af8ad
Signed-off-by: William Henrotin <Whenrow@users.noreply.github.com>