Some implementations, like bonding, has nest array with same attr type.
To support all kinds of entries under one nest array. As discussed[1],
let's rename array-nest to indexed-array, and assuming the value is
a nest by passing the type via sub-type.

[1] https://lore.kernel.org/netdev/20240312100105.16a59086@kernel.org/Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20240404063114.1221532-2-liuhangbin@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>