Support replicaPlacementFactory in solr.xml

[janhoy]

With https://issues.apache.org/jira/browse/SOLR-15782 in 9.5, we got support for configuring placement plugins in solr.xml:

<replicaPlacementFactory class="org.apache.solr.cluster.placement.plugins.AffinityPlacementFactory">
  <int name="minimalFreeDiskGB">10</int>
  <int name="prioritizedFreeDiskGB">200</int>
</replicaPlacementFactory>

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

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.