Handle legacy name in gen codec

There should be a way to handle other name for the field/class due to the compatibility reasons.

For example
```scala

@legacyName("B")
case class A(@legacyName("y") x: Int)
```

should parse all of
```json
[
"A" : {
    "x": 2
},
"B" : {
    "x": 2
},
"A" : {
    "y": 2
},
"B" : {
    "y": 2
},
]
```