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/context.rs

@@ -857,6 +857,12 @@ impl Context {
             "is_muted",
             self.get_config_bool(Config::IsMuted).await?.to_string(),
         );
+        res.insert(
+            "private_tag",
+            self.get_config(Config::PrivateTag)
+                .await?
+                .unwrap_or_else(|| "<unset>".to_string()),
+        );
 
         if let Some(metadata) = &*self.metadata.read().await {
             if let Some(comment) = &metadata.comment {