Support for user defined functions

Rust driver can read/write UDTs: https://rust-driver.docs.scylladb.com/stable/data-types/udt.html but seems that using it from latte workload is not currently possible passing in Rune object didn't worked: ```bash ❯ latte load docker/latte/workloads/custom_d1.rn -- 172.17.0.2 info: Loading workload script /home/fruch/projects/scylla-cluster-tests/docker/latte/workloads/custom_d1.rn... info: Connecting to ["172.17.0.2"]... info: Connected to running Cassandra version 3.0.8 info: Preparing... info: Erasing data... info: Loading data... error: Cassandra error: Unsupported type: Object ``` I'm guessing some constructs needs to be expose to Rune, for building needed UDT objects