service::client_state: add a const version of get_trace_state()
service::client_state: add a const version of get_trace_state()

tracing::begin() requires a non-const version, tracing::trace()
requires a const version.

Signed-off-by: Vlad Zolotarov <976c0f8d0852ed6439ad273880b400a75a7c41d0@cloudius-systems.com>

scylladb/scylla,scylladb/scylla,kjniemi/scylla,duarten/scylla,avikivity/scylla,kjniemi/scylla,avikivity/scylla,scylladb/scylla,raphaelsc/scylla,raphaelsc/scylla,kjniemi/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,raphaelsc/scylla,duarten/scylla