Provide a helper to perform Solr schema operations outside of the initializer

AbstractSolrCoreInitializer is providing a lot of API but those cannot be used outside of the initializer components. It would be useful to refactor properly all this: a first step has been done as part of XWIKI-20575 with an internal component but the design is not very good.
Also this helper should take into account that the search core as a schema that is different than other cores (e.g. some types do not exist).
tmortagne proposed that we provide a `SolrCoreSchema` interface allowing to perform the operations with different implem.