cql3: enable parsing decimal JSON values from string
cql3: enable parsing decimal JSON values from string

In order to be Cassandra-compatible, decimal type should be parsable
from both numeric values and strings.

Fixes #3666

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