[patch] add TTL as defcustom to ox-icalendar

[Detlef Steuer]

This is my first try ever to contribute anything in elisp.

The nextcloud calendar app supports the X-PUBLISHED-TTL 
option if subscribing to an ics file. Nextcloud defaults
to a TTL of 1 week, what is much too long for my use case.

When exporting to such a file from org I did not find any way
to set this option during export. So I tried to add a defcustom
in ox-icalendar.el.

The default TTL in this patch is 1 hour.

I think this qualifies as an tiny patch, as it is mostly a docstring
:-), in case anybody finds this an useful addition.

The patch is relative to 9.5.1 from elpa.

All comments very much appreciated.

And a happy new year to you all
Detlef

patch.ttl
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