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.go

@@ -887,6 +887,7 @@ type Hostinfo struct {
 	UserspaceRouter opt.Bool       `json:",omitzero"` // if the client's subnet router is running in userspace (netstack) mode
 	AppConnector    opt.Bool       `json:",omitzero"` // if the client is running the app-connector service
 	ServicesHash    string         `json:",omitzero"` // opaque hash of the most recent list of tailnet services, change in hash indicates config should be fetched via c2n
+	PeerRelay       bool           `json:",omitzero"` // if the client is willing to relay traffic for other peers
 	ExitNodeID      StableNodeID   `json:",omitzero"` // the client’s selected exit node, empty when unselected.
 
 	// Location represents geographical location data about a