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>

ipn/ipnlocal/local.go

@@ -5671,6 +5671,10 @@ func (b *LocalBackend) applyPrefsToHostinfoLocked(hi *tailcfg.Hostinfo, prefs ip
 	}
 	hi.SSH_HostKeys = sshHostKeys
 
+	if buildfeatures.HasRelayServer {
+		hi.PeerRelay = prefs.RelayServerPort().Valid()
+	}
+
 	for _, f := range hookMaybeMutateHostinfoLocked {
 		f(b, hi, prefs)
 	}