net: bridge: multicast: add EHT host handling functions

Add functions to create, destroy and lookup an EHT host. These are
per-host entries contained in the eht_host_tree in net_bridge_port_group
which are used to store a list of all sources (S,G) entries joined for that
group by each host, the host's current filter mode and total number of
joined entries.
No functional changes yet, these would be used in later patches.

Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>