Add REST response serializer, transparent content negotiation

Dieses Issue hat noch niemand übernommen.

## Suggestion

We should...
1. Provide an extension point to allow contribution of response deserializers
2. Process automatic content negotiation to use the correct deserializer, similar to `BodyParser`.

## Use Cases

Currently, there is only support for `BodyParser`s that de-serialize REST request bodies to JSON. This means the Request-Response Lifecycle cannot be fully covered by a `BodyParser`, and requires lb4 users to manually serialize the controllers' response and prematurely return the response using `res.end` (see: #5168).

## Examples

For the extension point, a serializer could implement the following design:

```typescript
interface ResponseSerializer {
isSupported = (contentType: string) => boolean;
serialize = (body: any) => string
}
```

If need be, we can create a higher-level class the merges role of the existing `BodyParser` and the new `ResponseSerializer`. This has the benefit of requiring a single class to implement both serialization and deserialization. However, this adds complication to the LB4 design, and should probably be considered **after** implementing the `ResponseSerializer` extension point, so that the separation between the lower-level and higher-level extension points are clear. In the long-term, we may deprecate the lower level `BodyParser` and `ResponseSerializer` in favour of the "unified (de-)serializer".

## Acceptance criteria

TBD - will be filled by the team.

Beginnen Sie mit der Überprüfung des vorhandenen BodyParser-Erweiterungspunkts und des Request-Response Lifecycle und vergleichen Sie anschließend die vorgeschlagene ResponseSerializer-Schnittstelle mit den Anforderungen an die Content Negotiation. Die Akzeptanzkriterien des Issues sind weiterhin TBD; für den Abschluss sind daher vereinbarte Kriterien erforderlich, die den Beitrag eines Serializers, die automatische Auswahl und die Verarbeitung der Antwort abdecken.

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