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/modrinth/ModrinthPackIndex.cpp

@@ -130,6 +130,7 @@ auto Modrinth::loadIndexedPackVersion(QJsonObject &obj, QString preferred_hash_t
         file.loaders.append(loader.toString());
     }
     file.version = Json::requireString(obj, "name");
+    file.changelog = Json::requireString(obj, "changelog");
 
     auto files = Json::requireArray(obj, "files");
     int i = 0;