tailcfg: add peerRelay bool to hostinfo

This commit adds a bool named PeerRelay to Hostinfo, to identify the host's status of acting as a peer relay.
Considering the RelayServerPort number can be 0, I just made this a bool in stead of a port number. If the port
info is needed in future this would also help indicating if the port was set to 0 (meaning any port in peer relay
context).

Updates tailscale/corp#35862

Signed-off-by: KevinLiang10 <37811973+KevinLiang10@users.noreply.github.com>

tailcfg/tailcfg_test.go

@@ -67,6 +67,7 @@ func TestHostinfoEqual(t *testing.T) {
 		"UserspaceRouter",
 		"AppConnector",
 		"ServicesHash",
+		"PeerRelay",
 		"ExitNodeID",
 		"Location",
 		"TPM",
@@ -244,6 +245,16 @@ func TestHostinfoEqual(t *testing.T) {
 			&Hostinfo{AppConnector: opt.Bool("false")},
 			false,
 		},
+		{
+			&Hostinfo{PeerRelay: true},
+			&Hostinfo{PeerRelay: true},
+			true,
+		},
+		{
+			&Hostinfo{PeerRelay: true},
+			&Hostinfo{PeerRelay: false},
+			false,
+		},
 		{
 			&Hostinfo{ServicesHash: "73475cb40a568e8da8a045ced110137e159f890ac4da883b6b17dc651b3a8049"},
 			&Hostinfo{ServicesHash: "73475cb40a568e8da8a045ced110137e159f890ac4da883b6b17dc651b3a8049"},