Trigger sampling after gossip block is execution valid

[dapplion]

Issue Addressed

Resolve a PeerDAS todo

Proposed Changes

Wait for a gossip block to be execution valid before triggering sampling. Otherwise, the block might have invalid transactions and trigger us to ask peers for data that does not exist. However, a proposer could still include valid blob transactions and withhold data, causing the same situation. Since gossip blocks are proposer signature valid, we have some DOS protection.

I'm 50/50 on this change being necessary, thoughts?