Ensure light_client/updates endpoint returns spec compliant SSZ data (#7230)

Closes #7167


  - Ensure the fork digest is generated from ther light client updates attested header and not the signature slot
- Ensure the format of the SSZ response is spec compliant

common/eth2/src/types.rs

@@ -802,13 +802,13 @@ pub struct LightClientUpdatesQuery {
 }
 
 #[derive(Encode, Decode)]
-pub struct LightClientUpdateSszResponse {
-    pub response_chunk_len: Vec<u8>,
-    pub response_chunk: Vec<u8>,
+pub struct LightClientUpdateResponseChunk {
+    pub response_chunk_len: u64,
+    pub response_chunk: LightClientUpdateResponseChunkInner,
 }
 
 #[derive(Encode, Decode)]
-pub struct LightClientUpdateResponseChunk {
+pub struct LightClientUpdateResponseChunkInner {
     pub context: [u8; 4],
     pub payload: Vec<u8>,
 }