enrich system_views.pending_hints with hints sizes

I am happy to close this if somebody shows me how to get total size of all hints or all hints per particular node via JMX.

I could find StorageMetrics.totalHints but that is how many hints there are, not their sizes. We also have

org.apache.cassandra.metrics:type=HintedHandOffManager name=<MetricName>
org.apache.cassandra.metrics:type=HintsService name=<MetricName>

But that is again showing other metrics not sizes.

I would add two methods into HintsServiceMBean returning this. Seems to be very easy to do once we do CASSANDRA-19477.