Add session context outbounds as slice (#3356)

* Add session context outbounds as slice

slice is needed for dialer proxy where two outbounds work on top of each other
There are two sets of target addr for example
It also enable Xtls to correctly do splice copy by checking both outbounds are ready to do direct copy

* Fill outbound tag info

* Splice now checks capalibility from all outbounds

* Fix unit tests

common/session/context.go

@@ -51,13 +51,13 @@ func InboundFromContext(ctx context.Context) *Inbound {
 	return nil
 }
 
-func ContextWithOutbound(ctx context.Context, outbound *Outbound) context.Context {
-	return context.WithValue(ctx, outboundSessionKey, outbound)
+func ContextWithOutbounds(ctx context.Context, outbounds []*Outbound) context.Context {
+	return context.WithValue(ctx, outboundSessionKey, outbounds)
 }
 
-func OutboundFromContext(ctx context.Context) *Outbound {
-	if outbound, ok := ctx.Value(outboundSessionKey).(*Outbound); ok {
-		return outbound
+func OutboundsFromContext(ctx context.Context) []*Outbound {
+	if outbounds, ok := ctx.Value(outboundSessionKey).([]*Outbound); ok {
+		return outbounds
 	}
 	return nil
 }