Add vCard viewtype

[link2xt]

Once we add import (#5202) and export (#5203) of vCards, we can use export function to generate vCard for a single contact and attach it to share contact instead of just setting a draft to email address and name (https://support.delta.chat/t/add-contact-as-vcard/277).

It should be possible to set viewtype for outgoing messages to special vCard type and receive it as such.

Messages with vCard viewtype will need additional API to get name, image and email address so it can be displayed in chat inline and used in confirmation dialog ("do you want to start chatting with ...?" or something like this). Maybe an indication of whether the key is attached is needed.

Details of the issue should be refined once #5202 and #5203 is implemented.

There is a jCard RFC which specifies how to represent the whole vCard in JSON. We likely don't need and want simple UI-focused API, adding it here for references so nobody proposes to use it.