Provide an SslContextFactory to jetty-client

[rossabaker]

Without this, https requests fail out of the box. That's a hostile default.

java.lang.NullPointerException: Missing SslContextFactory
	at java.util.Objects.requireNonNull(Objects.java:228)
	at org.eclipse.jetty.io.ssl.SslClientConnectionFactory.<init>(SslClientConnectionFactory.java:54)
	at org.eclipse.jetty.client.HttpClient.newSslClientConnectionFactory(HttpClient.java:1208)
	at org.eclipse.jetty.client.HttpClient.newSslClientConnectionFactory(HttpClient.java:1214)
	at org.eclipse.jetty.client.HttpDestination.newSslClientConnectionFactory(HttpDestination.java:149)
	at org.eclipse.jetty.client.HttpDestination.newSslClientConnectionFactory(HttpDestination.java:155)
	at org.eclipse.jetty.client.HttpDestination.<init>(HttpDestination.java:95)
	at org.eclipse.jetty.client.PoolingHttpDestination.<init>(PoolingHttpDestination.java:25)
	at org.eclipse.jetty.client.http.HttpDestinationOverHTTP.<init>(HttpDestinationOverHTTP.java:32)
	at org.eclipse.jetty.client.http.HttpClientTransportOverHTTP.newHttpDestination(HttpClientTransportOverHTTP.java:54)
	at org.eclipse.jetty.client.HttpClient.lambda$resolveDestination$0(HttpClient.java:575)
	at java.util.concurrent.ConcurrentHashMap.computeIfAbsent(ConcurrentHashMap.java:1660)
	at org.eclipse.jetty.client.HttpClient.resolveDestination(HttpClient.java:573)
	at org.eclipse.jetty.client.HttpClient.resolveDestination(HttpClient.java:551)
	at org.eclipse.jetty.client.HttpClient.send(HttpClient.java:599)
	at org.eclipse.jetty.client.HttpRequest.sendAsync(HttpRequest.java:778)
	at org.eclipse.jetty.client.HttpRequest.send(HttpRequest.java:765)
	at org.http4s.client.jetty.JettyClient$.$anonfun$allocate$8(JettyClient.scala:44)

This client doesn't follow the builder pattern of the other backends. If it did, it should take an SslContextFactory parameter.

The workaround until this is released is to pass a fully configured client. See the defaultHttpClient function for how.