feat: Delete vg-request-with-auth from IMAP after processing (#6208)

In multi-device case `vg-request-with-auth` left on IMAP may result in situation when Bob joins the
group, then leaves it, then second Alice device comes online and processes `vg-request-with-auth`
again and adds Bob back. So we should IMAP-delete `vg-request-with-auth`. Another device will know
the Bob's key from Autocrypt-Gossip. It's not a problem if Alice loses state (restores from an old
backup) or goes offline for long before sending `vg-member-added`, anyway it may not be delivered by
the server, rather Bob should retry sending SecureJoin messages as he is a part which wants to join,
so let's not solve this for now.

src/headerdef.rs

@@ -73,6 +73,7 @@ pub enum HeaderDef {
 
     /// [Autocrypt](https://autocrypt.org/) header.
     Autocrypt,
+    AutocryptGossip,
     AutocryptSetupMessage,
     SecureJoin,