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_view.go

@@ -606,6 +606,9 @@ func (v HostinfoView) AppConnector() opt.Bool { return v.ж.AppConnector }
 // opaque hash of the most recent list of tailnet services, change in hash indicates config should be fetched via c2n
 func (v HostinfoView) ServicesHash() string { return v.ж.ServicesHash }
 
+// if the client is willing to relay traffic for other peers
+func (v HostinfoView) PeerRelay() bool { return v.ж.PeerRelay }
+
 // the client’s selected exit node, empty when unselected.
 func (v HostinfoView) ExitNodeID() StableNodeID { return v.ж.ExitNodeID }
 
@@ -664,6 +667,7 @@ var _HostinfoViewNeedsRegeneration = Hostinfo(struct {
 	UserspaceRouter opt.Bool
 	AppConnector    opt.Bool
 	ServicesHash    string
+	PeerRelay       bool
 	ExitNodeID      StableNodeID
 	Location        *Location
 	TPM             *TPMInfo