implement Confirmation rule prerequisite - fork choice filter change

To support confirmation rule via beacon-APIs as described in spec PR,
add `--debug-fork-choice-version=pr3431` option and enable when Deneb
fork is scheduled. To opt-out, `--debug-fork-choice-version=stable`,
or don't schedule Deneb.

- ethereum/consensus-specs#3431
- ethereum/consensus-specs#3466

"will bundle this with deneb release":

- ethereum/pm#844 (comment)

beacon_chain/fork_choice/fork_choice_types.nim

@@ -33,6 +33,8 @@ type
     ## Controls which version of fork choice to run.
     Stable = "stable"
       ## Use current version from stable Ethereum consensus specifications
+    Pr3431 = "pr3431"
+      ## https://github.com/ethereum/consensus-specs/pull/3431
 
   fcKind* = enum
     ## Fork Choice Error Kinds

beacon_chain/fork_choice/proto_array.nim

@@ -530,16 +530,24 @@ func nodeIsViableForHead(
     self.checkpoints.justified.epoch == GENESIS_EPOCH or
     node.checkpoints.justified.epoch == self.checkpoints.justified.epoch
 
-  # If the previous epoch is justified, the block should be pulled-up.
-  # In this case, check that unrealized justification is higher than the store
-  # and that the voting source is not more than two epochs ago
-  if not correctJustified and self.isPreviousEpochJustified and
-      node.bid.slot.epoch == self.currentEpoch:
-    let unrealized =
-      self.currentEpochTips.getOrDefault(nodeIdx, node.checkpoints)
-    correctJustified =
-      unrealized.justified.epoch >= self.checkpoints.justified.epoch and
-      node.checkpoints.justified.epoch + 2 >= self.currentEpoch
+  if not correctJustified:
+    case self.version
+    of ForkChoiceVersion.Stable:
+      # If the previous epoch is justified, the block should be pulled-up.
+      # In this case, check that unrealized justification is higher than the
+      # store and that the voting source is not more than two epochs ago
+      if self.isPreviousEpochJustified and
+          node.bid.slot.epoch == self.currentEpoch:
+        let unrealized =
+          self.currentEpochTips.getOrDefault(nodeIdx, node.checkpoints)
+        correctJustified =
+          unrealized.justified.epoch >= self.checkpoints.justified.epoch and
+          node.checkpoints.justified.epoch + 2 >= self.currentEpoch
+    of ForkChoiceVersion.Pr3431:
+      # The voting source should be either at the same height as the store's
+      # justified checkpoint or not more than two epochs ago
+      correctJustified =
+        node.checkpoints.justified.epoch + 2 >= self.currentEpoch
 
   return
     if not correctJustified:

beacon_chain/nimbus_beacon_node.nim

@@ -1903,7 +1903,11 @@ proc doRunBeaconNode(config: var BeaconNodeConf, rng: ref HmacDrbgContext) {.rai
   for node in metadata.bootstrapNodes:
     config.bootstrapNodes.add node
   if config.forkChoiceVersion.isNone:
-    config.forkChoiceVersion = some(ForkChoiceVersion.Stable)
+    config.forkChoiceVersion =
+      if metadata.cfg.DENEB_FORK_EPOCH != FAR_FUTURE_EPOCH:
+        some(ForkChoiceVersion.Pr3431)
+      else:
+        some(ForkChoiceVersion.Stable)
 
   ## Ctrl+C handling
   proc controlCHandler() {.noconv.} =