Implement `Connection::get_statistics()`

This will also require querying the server.

https://docs.rs/adbc_core/0.21.0/adbc_core/trait.Connection.html#tymethod.get_statistics

Pre-defined statistics we need to support are listed by this enum: https://docs.rs/adbc_core/0.21.0/adbc_core/options/enum.Statistics.html

We can extend this later with additional statistics by implementing `Connection::get_statistic_names()`: https://docs.rs/adbc_core/0.21.0/adbc_core/trait.Connection.html#tymethod.get_statistic_names