[18.0][ADD] fieldservice_appointment_confirmation

[edescalona]

@BinhexTeam
T24082

This module automates the confirmation of field service appointments with the final customer.

From a field service order the coordinator can request a confirmation from the customer through a pre-filled, editable email (standard mail composer).

The customer confirms or reschedules the visit from a secure tokenized link, without any login or portal user, and the order is updated accordingly:

• the appointment status is tracked (Pending, Requested, Confirmed) with a badge, a ribbon and calendar/list decorations; a confirmed appointment that was rescheduled is shown as "Confirmed (rescheduled)";
• confirming and rescheduling are logged in the chatter and notify the order followers and the assigned worker; the customer also receives a confirmation email;
• picking an available slot when rescheduling is treated as the customer's acceptance, so the appointment is automatically confirmed on the new date;
• on confirmation (or reschedule) the customer can download an RFC 5545 .ics file to add the visit to Google Calendar, Outlook or Apple Calendar;
• rescheduling only offers slots where the assigned worker is actually available, based on their working schedule and existing orders;
• an appointment whose date has already passed can no longer be confirmed online, only rescheduled to a future slot.