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_clone.go

@@ -186,6 +186,7 @@ var _HostinfoCloneNeedsRegeneration = Hostinfo(struct {
 	UserspaceRouter opt.Bool
 	AppConnector    opt.Bool
 	ServicesHash    string
+	PeerRelay       bool
 	ExitNodeID      StableNodeID
 	Location        *Location
 	TPM             *TPMInfo