ipn, ipn/ipnlocal: add Foreground field to ServeConfig

This PR adds a new field to the ServeConfig which maps
WatchIPNBus session ids to foreground serve configs.

The PR also adds a DeleteForegroundSession method to ensure the config
gets cleaned up on sessions ending.

Note this field is not currently used but will be in follow up work.

Updates #8489

Signed-off-by: Marwan Sulaiman <marwan@tailscale.com>

ipn/ipn_clone.go

@@ -76,6 +76,12 @@ func (src *ServeConfig) Clone() *ServeConfig {
 		}
 	}
 	dst.AllowFunnel = maps.Clone(src.AllowFunnel)
+	if dst.Foreground != nil {
+		dst.Foreground = map[string]*ServeConfig{}
+		for k, v := range src.Foreground {
+			dst.Foreground[k] = v.Clone()
+		}
+	}
 	return dst
 }
 
@@ -84,6 +90,7 @@ var _ServeConfigCloneNeedsRegeneration = ServeConfig(struct {
 	TCP         map[uint16]*TCPPortHandler
 	Web         map[HostPort]*WebServerConfig
 	AllowFunnel map[HostPort]bool
+	Foreground  map[string]*ServeConfig
 }{})
 
 // Clone makes a deep copy of TCPPortHandler.