This series adds WCU support for the Alternator update item.
This motivation behind it, is to have a rough estimation of what a similar operation would have taken from WCU perspective if used with DynamoDB.
The calculation is done while minimal overhead is the prime objective, the results are values that is less or equal to what it would have been in DynamoDB
** New feature, no need to backport. **
Closesscylladb/scylladb#21999
* github.com:scylladb/scylladb:
alternator/test_returnconsumedcapacity.py: update item
alternator/executor.cc: Add WCU for update_item