networkctl: add support to display learned fdb entries

networkctl: add support to display learned fdb entries

Since Linux commit ddd1ad68826d ("net: bridge: Add netlink knobs for number
/ max learned FDB entries") [1] it is possible to limit to number of
dynamically learned fdb entries per bridge.

Add support to the systemd networkctl for the netlink bridge attributes
IFLA_BR_FDB_MAX_LEARNED and IFLA_BR_FDB_N_LEARNED.

[1] https://lore.kernel.org/all/20231016-fdb_limit-v5-0-32cddff87758@avm.de/

Signed-off-by: Gregor Herburger <gregor.herburger@ew.tq-group.com>