LogStage: support nested Messages, support interpolating Message inside of another Message

This:

```scala
val a = 1
val b = 2
val c = 3
val d = 4

IzLogger().log(s"Abc $a $b ${Message(s"$c $d")}")
```

Should produce log
```
Abc a=1 b=2 c=3 d=4
```

Instead of failing with compile error "Expression as a logger argument"