Update the voluntary exit endpoint to comply with the key manager specification (#4679)

## Issue Addressed

#4635 

## Proposed Changes

Wrap the `SignedVoluntaryExit` object in a `GenericResponse` container, adding an additional `data` layer, to ensure compliance with the key manager API specification.

The new response would look like this:

```json
{"data":{"message":{"epoch":"196868","validator_index":"505597"},"signature":"0xhexsig"}}
```

This is a backward incompatible change and will affect Siren as well.

common/eth2/src/lighthouse_vc/http_client.rs

@@ -666,7 +666,7 @@ impl ValidatorClientHttpClient {
         &self,
         pubkey: &PublicKeyBytes,
         epoch: Option<Epoch>,
-    ) -> Result<SignedVoluntaryExit, Error> {
+    ) -> Result<GenericResponse<SignedVoluntaryExit>, Error> {
         let mut path = self.server.full.clone();
 
         path.path_segments_mut()