[FIX] google_calendar: user invited to one recurring event occurence only

When a Google user was invited to one occurrence of a recurring event,
but not to the recurring event itself (and the other occurrences, therefore),
and the user owner of the recurring event did not sync his calendar in Odoo,
the event occurrence could not be synced in Odoo, because it attempted
to attach it to the parent/main recurring event, which was not present
in Odoo.

In such a case, Odoo now syncs the event occurrence as a simple classic
event. In addition, if the owner of the event sync his calendar with Odoo
afterwards, or if the user is invited later to the main/parent recurring event
and then sync again his calendar,
it then attach the event occurrence that was previously
synced to this main event, to avoid events duplication.

opw-676535