mgr/DaemonServer: Modify offline_pg_report to handle set or vector types

mgr/DaemonServer: Modify offline_pg_report to handle set or vector types

The offline_pg_report structure to be used by both the 'ok-to-stop' and
'ok-to-upgrade' commands is modified to handle either std::set or std::vector
type containers. This is necessitated due to the differences in the way
both commands work. For the 'ok-to-upgrade' command logic to work optimally,
the items in the specified crush bucket including items found in the subtree
must be strictly ordered. The earlier std::set container re-orders the items
upon insertion by sorting the items which results in the offline pg check to
report sub-optimal results.

Therefore, the offline_pg_report struct is modified to use
std::variant<std::vector<int>, std::set<int>> as a ContainerType and handled
accordingly in dump() using std::visit(). This ensures backward compatibility
with the existing 'ok-to-stop' command while catering to the requirements of
the new 'ok-to-upgrade' command.

Signed-off-by: Sridhar Seshasayee <sseshasa@redhat.com>