Add REST response serializer, transparent content negotiation

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## 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.

Inizia esaminando il punto di estensione esistente di BodyParser e il Request-Response Lifecycle, quindi confronta l’interfaccia ResponseSerializer proposta con i requisiti di negoziazione dei contenuti. I criteri di accettazione dell’issue sono ancora TBD, quindi il completamento richiede criteri concordati che coprano il contributo del serializer, la selezione automatica e la gestione della risposta.

Scritto dal modello di indicizzazione a partire dal testo della issue.