Enable splice for freedom outbound (downlink only)

- Add outbound name
- Add outbound conn in ctx
- Refactor splice: it can be turn on from all inbounds and outbounds
- Refactor splice: Add splice copy to vless inbound
- Fix http error test
- Add freedom splice toggle via env var
- Populate outbound obj in context
- Use CanSpliceCopy to mark a connection
- Turn off splice by default

proxy/trojan/client.go

@@ -54,6 +54,11 @@ func (c *Client) Process(ctx context.Context, link *transport.Link, dialer inter
 	if outbound == nil || !outbound.Target.IsValid() {
 		return newError("target not specified")
 	}
+	outbound.Name = "trojan"
+	inbound := session.InboundFromContext(ctx)
+	if inbound != nil {
+		inbound.SetCanSpliceCopy(3)
+	}
 	destination := outbound.Target
 	network := destination.Network