[QUESTION] How to conditionally omit a field during serialization?

Hi,

What's the nicest way to conditionally omit a field during serialization?

Let's say I have a class:
```kt
class MyDTO (
@JSONField(name = "id")
id: Int,

@JSONField(name = "key")
key: OmittableString
)
```

```kt
class OmittableString {
value: String?
omit: boolean
}
```

And based on values in `OmittableString` it can serialize to one of:

A. Value is set:
```json
{
"id": 123,
"key": "value"
}
```

B. Value is set to null:
```json
{
"id": 123,
"key": null
}
```

C. Value is omitted:
```json
{
"id": 123
}
```

I would like to write a serializer for `OmittableString` that can also do the option C, but on first look, it seems to me that `JSONWriter` is not the right place to look at, as it's responsible **only** for the value and not the key.

---

_Context: I'm serializing a request for a 3rd party API, and they use null value to patch existing value to null, and omit field to skip changing existing value._

Thanks in advance!

Start with JSONWriter, which the issue identifies as handling only the value, and trace how the key is emitted for @JSONField members. The work is done when OmittableString can serialize as a value, null, or an omitted key while preserving the id and key names.

Written by the indexing model from the issue text.