feat: allow to specify a reference to iterate on beacon anchor

[leruaa]

In this PR:

• In the host:
  • Update the SketchBuilder to allow setting a reference block. This block is a child of the execution block.
  • During anchor building, we walk backward from the reference block to the execution block, constucting a chain of the beacon root states
• In the client:
  • We iterate the beacon state anchors constructed in the host and verify them
  • The timestamp and beacon_root in the ResolvedAnchor are the ones correspondiong to the execution block.