Refactoring nftables package

Refactored tcpip/nftables/ and its contents to exist under tcpip/stack/ in
anticipation of implementing netlink netfilter sockets. These sockets need to
reference the system-wide nftables structure, which requires exposing it to all
processes.

The refactoring was necessary due to circular dependencies between the nftables
package and the stack package. hook_string.go was also regenerated to
include the ingress and egress hook enums.

nftables/nftables.go was split up accordingly to more easily manage its types
(nftables_types.go) and the byte operations within it (see nft_{op}.go files).

NOTE: Files that previously existed under the nftables package are still not
thread safe. Future changes will add synchronization primitives to resolve this
issue.
PiperOrigin-RevId: 766391911

Author: kerumeto