SolrJ does not use HTTP(S) proxy

HTTP/2-based Solr clients (Http2SolrClient etc) cannot be configured to use a proxy. For HTTP/1 clients this was solved via SOLR-12848. But environment variables http.proxyHost etc are not applied by Jetty clients which are used for HTTP/2 communication. Configuration of the Jetty client through org.eclipse.jetty.client.HttpClient#getProxyConfiguration() is not possible, either, since Http2SolrClient#getHttpClient() is package-protected.