Add support for sync-tocken or ctag in order to be able synchronize Caldav calendar with Android terminal

Hi all,

First of all, let me thank you for all the work done on GLPI.

I come here today in order to suggest an improvement on Caldav calendar support.
In order to explain, we are trying to share technician calendar using caldav.
It works fine with Thunderbird, but we'd like to include them in their Android Calendar.

We are using one of the very few free calendar app, One Calendar
https://play.google.com/store/apps/details?id=biz.codespark.xcalendarapp&hl=es_419&gl=US&pli=1

But unfortunately, when we try to add a calendar, we have an error  saying that the sync-tocken or ctag are not supported by he server.
I don't know if somebody, has found a free working App on Android?
Or otherwise if sync-tocken or ctag support could be added to GLPI.

Thanks for all
Best

Damien