Support replicaPlacementFactory in solr.xml

With https://issues.apache.org/jira/browse/SOLR-15782 in 9.5, we got support for [configuring placement plugins in solr.xml](https://solr.apache.org/guide/solr/latest/configuration-guide/configuring-solr-xml.html#the-replicaplacementfactory-element):

```xml

10
200

```

The operator could support a similar configuration option. Perhaps it can be made generic:

```yaml
replicaPlacementFactory:
class: org.apache.solr.cluster.placement.plugins.AffinityPlacementFactory
config:
minimalFreeDiskGB: 10
prioritizedFreeDiskGB: 200
```

I think we could add support for it before the minimum version is 9.5, since if the user does not configure the feature, nothing will be added to `solr.xml`. This can be documented.

Start with the linked Solr configuration guide and the existing solr.xml generation path in the operator. Compare how current configuration options are represented in YAML and emitted to solr.xml, then determine how the replicaPlacementFactory class and config should be handled. Done means the option is supported without changing output when omitted and the behavior is documented.

Written by the indexing model from the issue text.