Add an annotation to ignore properties/getters in `@DataSchema`

[Jolanrensen]

Writing something like

@DataSchema
data class KeyValue(val key: String, val value: String) {

    @Deprecated("use key, not name")
    val name get() = key
}

will cause column accessors to be generated for "name" as well as for "key" and "value", even though name is just a getter, planned to be removed. We don't want it as part of the DataFrame or the DataSchema; it holds no data (as found in the wild here #1532 (comment)).

Since we have a way to rename the underlying columns of properties, does it make sense to add support for something like @Transient to ignore properties as part of the schema?
It would then also be ignored when calling List<KeyValue>.toDataFrame().

We would need support from the compiler plugin and the older gradle plugin probably