Add UDT support for prepared statements

[vponomaryov]

Update the latte's to_scylla_value function (which is used by the execute_prepared one) to support CQL UserDefinedTypes.

If we create UDT like following:

  let KEYSPACE = "fooks"
  let UDT_NAME = "fooudt"
  context.execute(`CREATE TYPE IF NOT EXISTS ${KEYSPACE}.${UDT_NAME} (
    id bigint, textk1 text, textk2 text)`).await?;

And then use it in some table.
Then, with this change, we can use the rune::runtime::Object object defined like following:

  let udtexample = #{
    "_keyspace": KEYSPACE, "_type_name": UDT_NAME,
    "id": fooid, "textk1": textv1, "textk2": textv2
  };

in an execute_prepared function.

Note that _keyspace and _type_name are special and required fields which are used to satisfy the Scylla Rust driver's CqlValue::UserDefinedType object structure:

  pub enum CqlValue {
    ...
    UserDefinedType {
        keyspace: String,
        type_name: String,
        /// Order of `fields` vector must match the order of fields as defined in the UDT. The
        /// driver does not check it by itself, so incorrect data will be written if the order is
        /// wrong.
        fields: Vec<(String, Option<CqlValue>)>,
    },
    ...

Also, note that for proper workability of various sub-types of the UDT
such as bigint for the example id field mentioned above need to use
Scylla-rust driver 0.11 or newer.

Closes: #57

[vponomaryov]

Valueable to consider it with the scylla-rust-driver latest version:

• Bump scylla-rust-driver version #62