Implement Subnet Sampling for PeerDAS (#6410)

* Add `SAMPLES_PER_SLOT` config.

* Rename `sampling` module to `peer_sampling`

* Implement subnet sampling.

* Update lookup test.

* Merge branch 'unstable' into subnet-sampling

* Merge branch 'unstable' into subnet-sampling

# Conflicts:
#	beacon_node/beacon_chain/src/data_availability_checker.rs
#	beacon_node/http_api/src/publish_blocks.rs
#	beacon_node/lighthouse_network/src/types/globals.rs
#	beacon_node/network/src/sync/manager.rs

* Merge branch 'unstable' into subnet-sampling

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -3851,6 +3851,8 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
         }
 
         if let Some(data_columns) = data_columns {
+            // TODO(das): `available_block includes all sampled columns, but we only need to store
+            // custody columns. To be clarified in spec.
             if !data_columns.is_empty() {
                 debug!(
                     self.log, "Writing data_columns to store";