types: enable deserializing varint from JSON string
types: enable deserializing varint from JSON string

Previously deserialization failed because the JSON string
representing a number was unnecessarily quoted.

Fixes #3666
Message-Id: <f91fb168171f9f0233615788df9b8b88eb163a8f@scylladb.com>

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