perf(dav): skip non-calendar requests in webcal caching plugin

perf(dav): skip non-calendar requests in webcal caching plugin

The webcal caching plugin is active when the X-NC-CalDAV-Webcal-Caching header is there.

findPrincipalByUrl sends a request for /remote.php/dav/principals/users/admin/ using the caldavService which sets the header by default.[^1]

As X-NC-CalDAV-Webcal-Caching only impacts calendar requests, we can skip non-calendar requests.

[^1]: https://github.com/nextcloud/calendar/blob/b3670f1805ef9ef952d6abe4e5334e37b5a14133/src/services/caldavService.js#L43

Signed-off-by: Daniel Kesselberg <mail@danielkesselberg.de>