Parsing Decimal256

Since it seems to be missing from the data types example I wonder how one would parse a `Decimal(38, 18)` into a rust type. Is there a specific reason for this to be left out in the example?

Start by locating the data types example and reviewing how Decimal values are represented and parsed in the Rust client. Determine whether Decimal(38, 18) needs an example or an implementation change; done means the supported approach is documented clearly, with any relevant example or limitation covered.

Written by the indexing model from the issue text.