ipn/{ipnlocal,localapi},net/netkernelconf,client/tailscale,cmd/containerboot: optionally enable UDP GRO forwarding for containers (#12410)

Add a new TS_EXPERIMENTAL_ENABLE_FORWARDING_OPTIMIZATIONS env var
that can be set for tailscale/tailscale container running as
a subnet router or exit node to enable UDP GRO forwarding
for improved performance.
See https://tailscale.com/kb/1320/performance-best-practices#linux-optimizations-for-subnet-routers-and-exit-nodes
This is currently considered an experimental approach;
the configuration support is partially to allow further experimentation
with containerized environments to evaluate the performance
improvements.

Updates tailscale/tailscale#12295

Signed-off-by: Irbe Krumina <irbe@tailscale.com>

net/netkernelconf/netkernelconf_default.go

@@ -10,3 +10,9 @@ package netkernelconf
 func CheckUDPGROForwarding(tunInterface, defaultRouteInterface string) (warn, err error) {
 	return nil, nil
 }
+
+// SetUDPGROForwarding is unimplemented for non-Linux platforms. Refer to the
+// docstring in _linux.go.
+func SetUDPGROForwarding(tunInterface, defaultRouteInterface string) error {
+	return nil
+}