Add support for ScyllaDB's per partition rate limiting's new error

[Piotr Dulikowski]

The upcoming ScyllaDB 5.1 introduces a feature called per-partition rate limiting. In case the (user defined) per-partition rate limit is exceeded, the database will start returning a new kind of error, or fall back to Configuration_error if the driver does not support it.

Because the new error should be propagated to users and handled differently than other errors, the driver should be prepared to accept the new error.

Reference implementation in the scylla-rust-driver: scylladb/scylla-rust-driver#549

[Piotr Dulikowski]

Implemented in #108, closing.