feat: add changelog to mod providers

The Modrinth changelog is fairly straight-forward, as it's given to us
directly with the API call we already did. Flame, on the other hand,
requires us to do another call to get the changelog, so it can introduce
quite a heavy performance impact. This way, we make it optional to get
such changelog.

Signed-off-by: flow <flowlnlnln@gmail.com>

launcher/modplatform/ModIndex.h

@@ -62,6 +62,7 @@ struct IndexedVersion {
     QString hash_type;
     QString hash;
     bool is_preferred = true;
+    QString changelog;
 };
 
 struct ExtraPackData {