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

common/session/session.go

@@ -50,6 +50,16 @@ type Inbound struct {
 	Conn net.Conn
 	// Timer of the inbound buf copier. May be nil.
 	Timer *signal.ActivityTimer
+	// CanSpliceCopy is a property for this connection, set by both inbound and outbound
+	// 1 = can, 2 = after processing protocol info should be able to, 3 = cannot
+	CanSpliceCopy int
+}
+
+func(i *Inbound) SetCanSpliceCopy(canSpliceCopy int) int {
+	if canSpliceCopy > i.CanSpliceCopy {
+		i.CanSpliceCopy = canSpliceCopy
+	}
+	return i.CanSpliceCopy
 }
 
 // Outbound is the metadata of an outbound connection.
@@ -60,6 +70,10 @@ type Outbound struct {
 	RouteTarget    net.Destination
 	// Gateway address
 	Gateway net.Address
+	// Name of the outbound proxy that handles the connection.
+	Name string
+	// Conn is actually internet.Connection. May be nil. It is currently nil for outbound with proxySettings 
+	Conn net.Conn
 }
 
 // SniffingRequest controls the behavior of content sniffing.