Add a Virtual table to expose Streaming metrics

The streaming metrics for the different nodes should be exposed through a virtual table.

Additional information for newcomers:

A new class should be created for representing the new Virtual Table in org.apache.cassandra.db.virtual. You can look at ThreadPoolsTable for an example of virtual table implementation and to TableMetricTables#buildMetadata for how Timer metrics can be exposed. The new table will need to be registered in SystemViewsKeyspace.
For the unit tests the easiest way is to add them at StreamingMetrics.