[IMP] mail: allow auto_commit parameter with templated message posts

The underlying `mail.compose.message` model has a useful `auto_commit`
argument when sending emails, which is unavailable when posting messages with
template through message_post_with_template.

This patch simply exposes the preexisting feature, to ease developments that
need it.

closes odoo/odoo#37783

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>