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.

[HoustonPutman]

I really wish that Solr accepted "additions" to the solr.xml. So a user could provide extra settings that would "override" the default solr.xml. That way they wouldn't have to maintain an entire solr.xml just to provide a replicaPlacementFactory.

[dsmiley]

"additions" -- not the word that comes to mind; I think of "overlay" or "override". I raised this topic on the dev list for solrconfig.xml in 2021 and also mentioned solr.xml too https://lists.apache.org/thread/nf2vgoqpdl5o718hovvq7xnppcp6j37m