Allow custom events with X-ABDATE property

Google uses X-ABDATE extension in its vcard on gmail to manage generic dates (events).
The format is the same and already managed, for the phone number, by nextcloud.

phone number e.g. (working)
item1.TEL:0123456789
item1.X-ABLABEL:foo

X-ABDATE e.g. (not working)
item2.X-ABDATE:1997-10-02
item2.X-ABLABEL:bar

If I export a contact from google and import it in nextcloud, the X-ABDATE information remain in the vcard but isn't shown in the frontend.
I think it would be nice to be able to see this information.

It is also possible to import it into android by using the specific extension:
X-ANDROID-CUSTOM:vnd.android.cursor.item/contact_event;1997-10-02;0;bar;;;;;;;;;;;;
This extension works from android 4 to 7. I personally tested it.

I hope you decide to implement this extension.

Cheers

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No files or tests are named in the issue. Start by tracing the contact import and frontend display paths for vCards, then inspect how X-ABDATE and X-ABLABEL are currently preserved or rendered. Done means imported X-ABDATE values appear as custom contact events with their labels, while existing phone-number extensions continue to work.