alternator: cleanup raw GetString() calls
alternator: cleanup raw GetString() calls

Instead of using raw GetString() from rapidjson, it's neater
to use a helper for creating string views: rjson::to_string_view().
Message-Id: <2d8c112020c14e103b64486b622e175f9259a529@scylladb.com>

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