Apache Solr channel adapters & gateways support

[markusherbert]

Expected Behavior There should be support for Reactive and non Reactive code (with .block()). Solr also mostly prefers bulk operations, therefore there should be native support for this (Reactively with .block()). Spring Data Solr should not get used - we should use SolrJ for this.

Current Behavior No native support (HTTP support can help though)

Context The reason for not using Spring Data Solr.