Relates #77466

Unlike the mapping metadata which we store in compressed+serialized for on this master node, we do store the settings for each index as deserialized Settings object. This can be a significant source of heap usage if there's a large number of indices with non-trivial settings. An example of this would be the audit-beats template which contains a fairly long list of field names in index.query.default_field. In this example, handling 10k audit-beat indices takes almost 500MB of master heap just for storing the duplicate lists of field names in Settings instances.

I will look for an easy win here, it shouldn't be too hard to deduplicate these in some form when building index metadata.