Add VK (VKontakte) iOS Messenger support

[abrignoni]

Adds VK (VKontakte) support for the iOS client, which had no coverage.

Two artifacts from the client's per-account VKM_ stores under Application Support:

• VK - Messages: read from the message table of messages-store.sqlite, whose data column holds each message as JSON in the clear. Reads the sent time, text, from id, out direction, forwarded count, attachment types and reactions. The peer is resolved to a conversation title from the peer_title table in the same store, and the sender is resolved to a name from the sibling content-store.sqlite. Adds a conversation view.
• VK - Users: read from the user table of content-store.sqlite (id, first and last name, screen name, nickname, sex, verified, closed).

Attachment bytes are cached separately by the client and are not rendered here; the attachment types are listed instead. Field mapping was done against a private sample.