Currently, because serialize_visitor::operator() is not implemented for counters, we cannot convert a counter returned by a WASM UDF to bytes when returning from wasm::run_script().
We could disallow using counters as WASM UDF return types, but an easier solution which we're already using in Lua UDFs is treating the returned counters as 64-bit integers when deserializing. This patch implements the latter approach and adds a test for it.
Closes#12806
* github.com:scylladb/scylladb:
wasm udf: deserialize counters as integers
test_wasm.py: add utility function for reading WASM UDF saved in files
310638e84d