Allow to list all caches from the Cache Service

In order to improve the monitoring and management of XWiki instances, it would be great to have an API to retrieve the list of cache instances opened using the XWiki Cache Service. The cache API should also include a way to read the cache size setting of each of the cache and the number of items included in each cache. Finally when flushing the cache we could fluch it using this list instead of relying to calls to XWiki Services and plugins to flush each of their cache.