Add support for custom serialization names

[JorgeCastilloPrz]

Ideally you'd be able to provide custom serialization names as you do for other libraries like:

data class MyClass(
    @SerializedName("user_id") val id: String,
    @SerializedName("user_name") val name: String,
    val lastName: String,
    val mail: String
)

That helps when you want to name your properties differently than in the json, which is a common use case.