[#3770] Add client-classes to host back end

[#3770] Add client-classes to host back end

/src/hooks/dhcp/mysql/mysql_host_data_source.cc
/src/hooks/dhcp/pgsql/pgsql_host_data_source.cc
    Set and fetch host options' client-classes

/src/lib/dhcpsrv/testutils/generic_host_data_source_unittest.cc
    Update tests

/src/share/database/scripts/mysql/upgrade_030_to_031.sh.in
/src/share/database/scripts/mysql/dhcpdb_create.mysql
/src/share/database/scripts/pgsql/dhcpdb_create.pgsql
/src/share/database/scripts/pgsql/upgrade_029_to_030.sh.in
    update dhcp6_options table