core: add grop member role "observer" (#1868)

* core: add grop member role "observer"

* disable observer role until supported by most clients

src/Simplex/Chat/Controller.hs

@@ -657,7 +657,7 @@ data ChatErrorType
   | CEContactNotReady {contact :: Contact}
   | CEContactDisabled {contact :: Contact}
   | CEConnectionDisabled {connection :: Connection}
-  | CEGroupUserRole
+  | CEGroupUserRole {requiredRole :: GroupMemberRole}
   | CEContactIncognitoCantInvite
   | CEGroupIncognitoCantInvite
   | CEGroupContactRole {contactName :: ContactName}