tailcfg: add Hostinfo.ExitNodeID to report the selected exit node (#16625)

When a client selects a particular exit node, Control may use that as
a signal for deciding other routes.

This patch causes the client to report whenever the current exit node
changes, through tailcfg.Hostinfo.ExitNodeID. It relies on a properly
set ipn.Prefs.ExitNodeID, which should already be resolved by
`tailscale set`.

Updates tailscale/corp#30536

Signed-off-by: Simon Law <sfllaw@tailscale.com>

tailcfg/tailcfg_view.go

@@ -300,6 +300,7 @@ func (v HostinfoView) Userspace() opt.Bool                    { return v.ж.User
 func (v HostinfoView) UserspaceRouter() opt.Bool              { return v.ж.UserspaceRouter }
 func (v HostinfoView) AppConnector() opt.Bool                 { return v.ж.AppConnector }
 func (v HostinfoView) ServicesHash() string                   { return v.ж.ServicesHash }
+func (v HostinfoView) ExitNodeID() StableNodeID               { return v.ж.ExitNodeID }
 func (v HostinfoView) Location() LocationView                 { return v.ж.Location.View() }
 func (v HostinfoView) TPM() views.ValuePointer[TPMInfo]       { return views.ValuePointerOf(v.ж.TPM) }
 
@@ -345,6 +346,7 @@ var _HostinfoViewNeedsRegeneration = Hostinfo(struct {
 	UserspaceRouter opt.Bool
 	AppConnector    opt.Bool
 	ServicesHash    string
+	ExitNodeID      StableNodeID
 	Location        *Location
 	TPM             *TPMInfo
 	StateEncrypted  opt.Bool