Add a virtual table for exposing prepared statements metrics

There are some existing metrics within CQLMetrics that provide metrics about prepared statements. We should expose those 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.
The new table will need to be registered in SystemViewsKeyspace.
Some unit tests will need to be added. For some example of virtual table tests you can look at SystemPropertiesTableTest and for some example of test around prepared statement metrics you can look into CQLMetricsTest.