sql: expose a virtual table with cockroachdb-specific version information

[knz]

Currently our SQL-level version reporting only advertises "this is postgresql".
There is no way for a SQL client to be sure that they are connected to CockroachDB.

There should be a feature for clients to check whether they are running against CockroachDB.

Currently this can be approximated by running show tables from crdb_internal but that only says "this si CockroachDB" and does not report the version.