tailcfg,hostinfo: add App field to identify tsnet uses

This allows us to differentiate between the various tsnet apps that
we have like `golinks` and `k8s-operator`.

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

tailcfg/tailcfg_test.go

@@ -41,6 +41,7 @@ func TestHostinfoEqual(t *testing.T) {
 		"Distro",
 		"DistroVersion",
 		"DistroCodeName",
+		"App",
 		"Desktop",
 		"Package",
 		"DeviceModel",
@@ -216,6 +217,16 @@ func TestHostinfoEqual(t *testing.T) {
 			&Hostinfo{},
 			false,
 		},
+		{
+			&Hostinfo{App: "golink"},
+			&Hostinfo{App: "abc"},
+			false,
+		},
+		{
+			&Hostinfo{App: "golink"},
+			&Hostinfo{App: "golink"},
+			true,
+		},
 	}
 	for i, tt := range tests {
 		got := tt.a.Equal(tt.b)