add LightClientHeader wrapper

The light client data structures were changed to accommodate additional
fields in future forks (e.g., to also hold execution data).

There is a minor change to the JSON serialization, where the `header`
properties are now nested inside a `LightClientHeader`.
The SSZ serialization remains compatible.

See ethereum/consensus-specs#3190

types/altair/light_client.yaml

@@ -21,11 +21,17 @@ Altair:
     minItems: 5
     maxItems: 5
 
+  LightClientHeader:
+    type: object
+    properties:
+      beacon:
+        $ref: '../block.yaml#/BeaconBlockHeader'
+
   LightClientBootstrap:
     type: object
     properties:
       header:
-        $ref: '../block.yaml#/BeaconBlockHeader'
+        $ref: '#/LightClientHeader'
       current_sync_committee:
         $ref: './sync_committee.yaml#/Altair/SyncCommittee'
       current_sync_committee_branch:
@@ -34,13 +40,13 @@ Altair:
     type: object
     properties:
       attested_header:
-        $ref: '../block.yaml#/BeaconBlockHeader'
+        $ref: '#/LightClientHeader'
       next_sync_committee:
         $ref: './sync_committee.yaml#/Altair/SyncCommittee'
       next_sync_committee_branch:
         $ref: '#/Altair/NextSyncCommitteeBranch'
       finalized_header:
-        $ref: '../block.yaml#/BeaconBlockHeader'
+        $ref: '#/LightClientHeader'
       finality_branch:
         $ref: '#/Altair/FinalityBranch'
       sync_aggregate:
@@ -51,9 +57,9 @@ Altair:
     type: object
     properties:
       attested_header:
-        $ref: '../block.yaml#/BeaconBlockHeader'
+        $ref: '#/LightClientHeader'
       finalized_header:
-        $ref: '../block.yaml#/BeaconBlockHeader'
+        $ref: '#/LightClientHeader'
       finality_branch:
         $ref: '#/Altair/FinalityBranch'
       sync_aggregate:
@@ -64,7 +70,7 @@ Altair:
     type: object
     properties:
       attested_header:
-        $ref: '../block.yaml#/BeaconBlockHeader'
+        $ref: '#/LightClientHeader'
       sync_aggregate:
         $ref: './sync_aggregate.yaml#/Altair/SyncAggregate'
       signature_slot: