rtnetlink: Add native SR-IOV VF status

Add a transport neutral kernel snapshot for NIC-specific SR-IOV VF
status and an optional iflib provider method. Providers gather state
under driver defined s

rtnetlink: Add native SR-IOV VF status

Add a transport neutral kernel snapshot for NIC-specific SR-IOV VF
status and an optional iflib provider method. Providers gather state
under driver defined synchronization.

Honor RTEXT_FILTER_VF on RTM_GETLINK requests and encode the status as
native typed route Netlink attributes. Represent VFs, driver
namespaces, and namespace fields as directly repeated nested attributes.
Presence masks in consumers can distinguish omission from false or zero.

Drivers may add custom status under stable, versioned namespaces. The
named, typed representation lets generic transports and consumers carry
or display fields without knowing their driver-specific schemas, while
the driver retains ownership of their names and meanings.

Document the ABI and add parser and RTM_GETLINK coverage.

Reviewed by: melifaro, iflib (gallatin), kgalazka (previous version)
Sponsored by: BBOX.io
Differential Revision: https://reviews.freebsd.org/D58776

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