repair: add get_hosts_participating_in_repair
repair: add get_hosts_participating_in_repair

Adds the `get_hosts_participating_in_repair` function which returns a
list of hosts participating in repair.

This list will be used by repair coordinator to tell other nodes to wait
until they replay their hints towards the nodes from the list.

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