Implement is_virtual APIs for table and keyspace meta

[dkropachev]

APIs to cover:

cass_keyspace_meta_is_virtual
cass_table_meta_is_virtual

[Lorak-mmk]

@wprzytula IIRC, our conclusion was that Scylla does not expose this metadata (at least not in the same way that C* does), right?

[wprzytula]

@wprzytula IIRC, our conclusion was that Scylla does not expose this metadata (at least not in the same way that C* does), right?

Yes. This API is meaningless for ScyllaDB. Also, Rust Driver doesn't support this.

[Lorak-mmk]

I'll reopen this - other functions that are C*-specific are kept open (custom payload, udf, uda).
I was asking for ScyllaDB compat, because I'm reworking Limitations section in README, and I wanted to know which section this should go in.
There is no problem if e.g. some external contributor picks it up.