add a prosody module to insert identity information (when available) … (#2627)

* add a prosody module to insert identity information (when available) into
presence

prosody will check for jitsi_meet_context_user and
jitsi_meet_context_group in the session and, if they are present, insert
them into presence (we do this in prosody so they cannot be spoofed).

* remove unused 'presence' variable

* refactor to modify presence message in place

* make object member access consistent

* make the group information optional

resources/prosody-plugins/mod_presence_identity.lua

+local stanza = require "util.stanza";
+
+-- For all received presence messages, if the jitsi_meet_context_(user|group)
+-- values are set in the session, then insert them into the presence messages
+-- for that session.
+function on_message(event)
+    if event and event["stanza"] then
+      if event.origin and event.origin.jitsi_meet_context_user then
+          -- First remove any 'identity' element if it already
+          -- exists
+          event.stanza:maptags(
+              function(tag)
+                  for k, v in pairs(tag) do
+                      if k == "name" and v == "identity" then
+                          return nil
+                      end
+                  end
+                  return tag
+              end
+          )
+          module:log("debug", "Presence after previous identity stripped: %s", tostring(event.stanza))
+
+          event.stanza:tag("identity"):tag("user")
+          for k, v in pairs(event.origin.jitsi_meet_context_user) do
+              event.stanza:tag(k):text(v):up()
+          end
+          event.stanza:up()
+            
+          -- Add the group information if it is present
+          if event.origin.jitsi_meet_context_group then
+              event.stanza:tag("group"):text(event.origin.jitsi_meet_context_group)
+          end
+
+          module:log("debug", "Sending presence with identity inserted %s", tostring(event.stanza))
+      end
+    end
+end
+
+module:hook("pre-presence/bare", on_message);
+module:hook("pre-presence/full", on_message);