alternator: enable support for UpdateItem's UpdateExpression
alternator: enable support for UpdateItem's UpdateExpression

For the UpdateItem operation, so far we supported updates via the
AttributeUpdates parameter, specifying which attributes to set or remove
and how. But this parameter is considered deprecated, and DynamoDB supports
a more elaborate way to modify attributes, via an "UpdateExpression".

In the previous patch we added a function to parse such an UpdateExpression,
and in this patch we use the result of this parsing to actually perform
the required updates.

UpdateExpression is only partially supported after this patch. The basic
"SET" and "REMOVE" operations are supported, but various other cases aren't
fully supported and will be fixed in followup patches. The following
patch will add extensive tests to confirm exactly what works correctly
with the new UpdateExpression support.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla