Synchronize accepting not yet existing contacts

[link2xt]

Synchronization message for accepting or blocking a contact 1:1 chat may arrive before the first message, when the contact does not exist yet. This frequently happens in non-chatmail accounts that have moving to DeltaChat folder disabled because Delta Chat unconditionally uploads sync messages to DeltaChat folder (even over IMAP since #5901 is merged).

For chats identified by SyncId::ContactAddr, if contact does not exist yet Delta Chat should create a hidden contact and hidden (blocked) 1:1 chat, then accept it.

Handling group chats and even blocking is outside of the scope for the issue, this can be closed once Delta Chat handles reception of accept sync item for not yet existing 1:1 chat as it is the most common case.

This issue is similar to #5954

[iequidoo]

Made a fix, the only difference is that i create a Blocked::Request 1:1 chat, this doesn't matter if things go on normally, but e.g. if the program crashes, it's easier for the user to find the chat and accept/block it.