netlink: specs: add display-hint to schema definitions

netlink: specs: add display-hint to schema definitions

Add a display-hint property to the netlink schema that is for providing
optional hints to generic netlink clients about how to display attribute
values. A display-hint on an attribute definition is intended for
letting a client such as ynl know that, for example, a u32 should be
rendered as an ipv4 address. The display-hint enumeration includes a
small number of networking domain-specific value types.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20230623201928.14275-2-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>