SOLR-16017: Allow first registered SolrIndexSearcher to inform its configured caches

[magibney]

See: SOLR-16017

add a hook for initializing caches by passing a reference to the first registered searcher

This complements (and is analogous to) SolrCache.warm(...); together, the new bootstrap(...) method and the existing warm(...) method allow for consistency across the lifecycle of SolrCache instances.

There is an old comment about possibly passing SolrIndexSearcher in cache init(...), and having the cache retain a reference to the searcher. We prefer the approach taken here -- for now -- for backward compatibility, and so that any change in how SolrIndexSearcher references are passed to SolrCache instances may be considered as a discrete change.