tailcfg: define a type for NodeCapability

Instead of untyped string, add a type to identify these.

Updates #cleanup

Signed-off-by: Maisem Ali <maisem@tailscale.com>

wgengine/filter/filter_test.go

@@ -873,7 +873,7 @@ func TestMatchesMatchProtoAndIPsOnlyIfAllPorts(t *testing.T) {
 	}
 }
 
-func TestCaps(t *testing.T) {
+func TestPeerCaps(t *testing.T) {
 	mm, err := MatchesFromFilterRules([]tailcfg.FilterRule{
 		{
 			SrcIPs: []string{"*"},