feat: Resend the last 10 messages to new broadcast member (#8151)

Last 10 messages in a broadcast channel are resent. They are sent and encrypted only to the new member, not to other subscribers.

Close #7678

Based on https://github.com/chatmail/core/pull/7854, with the following
changes:

- Refactor and simplify code, don't reuse the existing `get_chat_msgs()`
function
- Document that Param::Arg4 is also used for resent messages
cc818d9099
- It's unclear how exactly to resend webxdc status updates. After
discussing with @r10s, don't resend webxdc's at all for now.
38d57ebb30
- Don't set fake `msg_id` in resent messages
e7d0687d90
Setting the msg_id to `u32::MAX` is hacky, and may just as well break
    things as it may fix things, because some code may use the msg.id to
    load information from the database, like `get_iroh_topic_for_msg()`.
    From reading the code, I couldn't find any problem with leaving the
    correct `msg_id`, and if there is one, then we should add a function
parameter `is_resending` that is checked in the corresponding places.

Easiest to review file-by-file rather than individual commits, probably.
I'll squash-merge this.

---------

Co-authored-by: iequidoo <dgreshilov@gmail.com>

src/param.rs

@@ -136,6 +136,10 @@ pub enum Param {
     /// For "MemberAddedToGroup" and "MemberRemovedFromGroup",
     /// this is the fingerprint added to / removed from the group.
     ///
+    /// For messages resent when adding a new member to a broadcast channel,
+    /// this is the fingerprint of the added member;
+    /// the message must only be sent to this one member then.
+    ///
     /// For call messages, this is the end timsetamp.
     Arg4 = b'H',