Add support for `shards.preference=replica.leader:false`

[magro]

Add support for shards.preference=replica.leader:false to prefer non-leader replicas.

See also http://issues.apache.org/jira/browse/SOLR-15472 and https://solr.apache.org/docs/8_10_0/changes/Changes.html#v8.10.0.new_features

This extends functionality added with #48