[FIX] calendar: use given attendee states at event creation

When an event is created from an external calendar account such as
Google or Outlook, attendee info such as email and state may be given,
and should be taken into account.

For example, if the current user who is syncing his calendar is not
the organizer of the event, his attendee state should be set to
'needsAction' and not automatically set to 'accepted'.

opw-2489815

closes odoo/odoo#79313

Signed-off-by: Arnaud Joset <arj@odoo.com>