Support couter proposals for invitations

[ChristophWurst]

Is your feature request related to a problem? Please describe.

As a Nextcloud user I receive invites to meetings. When I can't make it to the suggested time I can only decline the event, and have to reach out through other channels to tell the organizer an alternative time that would work for me.

Describe the solution you'd like

Allow me to enter a counter proposal right in calendar, with the option to decline too.

Describe alternatives you've considered

N/a

Additional context

• Requires Process COUNTER and DECLINECOUNTER for iTIP requests server#49314.