Let CloudHttp2SolrClient accept an external Http2SolrClient Builder

CloudHttp2SolrClient doesn't provide much of the options that Http2SolrClient does (timeouts, max connections per hosts, etc). Technically it accepts a fully built Http2SolrClient, however, in such case the client becomes "external", which means it won't be closed when the CloudClient is closed (one needs to maintain a reference and close explicitly after closing CloudClient).
CloudHttp2SolrClient will use an empty/default Http2SolrClient.Builder to build it's internal client. I propose we allow providing a configured Http2SolrClient.Builder instead and let the CloudHttp2SolrClient just build from it. This would be optional of course, and backwards compatible.