feat: internal profile names (#6088)

this PR allows setting a "private tag" for a profile, see
https://github.com/deltachat/deltachat-android/pull/3373 for a possible
UI.

currently, the core does not do anything with the tag (so, it could also
be a ui.-config option), however, this may change in the future - it
might bet synced, and become also otherwise useful in core. also, having
this in core is better documentation-wise, as otherwise each UI easily
does its own things :)

src/config.rs

@@ -321,6 +321,10 @@ pub enum Config {
     /// True if account is muted.
     IsMuted,
 
+    /// Optional tag as "Work", "Family".
+    /// Meant to help profile owner to differ between profiles with similar names.
+    PrivateTag,
+
     /// All secondary self addresses separated by spaces
     /// (`addr1@example.org addr2@example.org addr3@example.org`)
     SecondaryAddrs,