api: Unencrypted group creation (#6927)

[iequidoo]

Let's discuss the naming here. At the Core level we don't need two separate APIs, so create_group_ex() or smth similar should work. The question is the naming in json-rpc (and CFFI if we decide to add it). Alternatives:

• create_email_group_chat() or just create_email_chat() (the word "chat" is needed to make it clear that a chat is created, not a message. Actually chat ID is returned). We don't use the word "email" for this purpose in the code currently, but maybe we want to switch to it. I'm not involved in the discussions a lot, so can't say much. But my "external" opinion is that technically everything is email and using this word only for a subset of chats/messages would complicate undestanding the code for newcomers and external audit. But if we switch to "email", probably we need to get rid of "adhoc" in the code.
• create_adhoc_{group,chat}(). We already have "adhoc" in receive_imf at least.

For now i named it create_group_chat_unencrypted() because i'm used to namespace/prefix naming personally and there are already Chat::is_encrypted() in Core and {FullChat,BasicChat}::is_encrypted in json-rpc which exactly reflect the difference between create_group_chat() and the new function and i decided not to introduce new words.

Close #6927