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.

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

pkg/tcpip/BUILD

@@ -87,6 +87,11 @@ deps_test(
         "//pkg/sync/locking",
         "//pkg/waiter",
         "//pkg/xdp",
+        "//pkg/abi/linux",
+        "//pkg/abi:abi",
+        "//pkg/hostarch:hostarch",
+        "//pkg/marshal:marshal",
+        "//pkg/marshal/primitive:primitive",
 
         # Other deps.
         "@com_github_google_btree//:go_default_library",