[CAL-714] caldav speed test when adding

[PeerRich]

when adding a caldav calendar https://app.cal.com/apps/caldav-calendar run an initial speed test and ping the server.

if too slow, send a warning that you may wanna add a different server or upgrade your caldav server

partly fixes https://github.com/calcom/cal.com/issues/4535

_{From SyncLinear.com | CAL-714}

[mykeels]

I get 404 when I try to access https://cal.com/apps/caldav

[PeerRich]

whops, https://app.cal.com/apps/caldav-calendar is the correct URL

[PeerRich]

@alishaz-polymath do you think this is valuable adding somewhere to caldav? to check connection speed

[alishaz-polymath]

Yes. It's on the list of CalDAV experience improvement features I'm currently compiling. definitely useful specially once we've added a timeout/lazy load to CalDAV integration so that it doesn't break the entire app Store/calendar pages