Missing Proxy support for Http2SolrClient

We are currently trying to upgrade to Solr 9 and are using Http2SolrClient to connect to Solr.

Sadly the internal HttpClient doesn't pick up the system's proxy settings. Proxy support needs to be configured, but the client used by Http2SolrClient isn't exposed through the API so there is no way to set that configuration.

The class also cannot be extended so there really doesn't seem any way to make it work through a proxy anymore.

It would be nice if the Builder had something like withProxyConfiguration().