Allow to declare replica placement plugins in solr.xml

Currently, container-level plugins can only be declared installed via calls to /cluster/plugin APIs. In some situations (especially corporate ones having tight security requirements), it is not possible to mutate the configuration of a cluster at runtime. Cluster should rather be configured fully at startup, and any change in config requires a re-deployment.

While there are several affected plugins, this ticket is specifically about making replica placement plugins configurable in solr.xml