Process COUNTER and DECLINECOUNTER for iTIP requests

[ChristophWurst]

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

The iTIP protocol used by CalDAV methods for attendees to suggest a counter proposal for a meeting time: https://www.rfc-editor.org/rfc/rfc5546.html#page-18. This is currently not supported by our implementation.

Describe the solution you'd like

Implement handling of COUNTER and DECLINECOUNTER methods, ideally upstream in Sabre.

Describe alternatives you've considered

N/a

Additional context

N/a