display current fork (+ next fork if applicable) in slot start / status

Extend slot start message and default status bar with information about
current head fork and the next fork transition (corresponding to head).
This is useful to know whether a synced client is aware of a future fork
and can also be useful when syncing from old forks to follow progress
across the various forks.

```
 peers: 8 ❯ finalized: 741c2ce2:230474 ❯ head: b330f58b:230477:20 ❯ fork: Capella (next: Deneb:231680) ❯ time: 230599:24 (7379192) ❯ sync: 00h24m (99.63%) 2.6492slots/s (QwQUwQPQDQ:7375263)/opt
```

```
INF 2024-01-12 12:18:00.001+01:00 Slot start                                 topics="beacnde" slot=7379190 epoch=230599 fork="Capella (next: Deneb:231680)" sync="--h--m (99.62%) 0.0000slots/s (wwwwwwwwww:7375167)/opt" peers=0 head=741c2ce2:7375168 finalized=230472:723abe7e delay=1ms861us
```

beacon_chain/conf.nim

@@ -440,6 +440,7 @@ type
         defaultValue: "peers: $connected_peers;" &
                       "finalized: $finalized_root:$finalized_epoch;" &
                       "head: $head_root:$head_epoch:$head_epoch_slot;" &
+                      "fork: $consensus_fork;" &
                       "time: $epoch:$epoch_slot ($slot);" &
                       "sync: $sync_status|" &
                       "ETH: $attached_validators_balance"

beacon_chain/nimbus_beacon_node.nim

@@ -884,7 +884,7 @@ func forkDigests(node: BeaconNode): auto =
 # https://github.com/ethereum/consensus-specs/blob/v1.4.0-beta.5/specs/phase0/p2p-interface.md#attestation-subnet-subscription
 proc updateAttestationSubnetHandlers(node: BeaconNode, slot: Slot) =
   if node.gossipState.card == 0:
-    # When disconnected, updateGossipState is responsible for all things
+    # When disconnected, updateBlocksGossipStatus is responsible for all things
     # subnets - in particular, it will remove subscriptions on the edge where
     # we enter the disconnected state.
     return
@@ -1507,6 +1507,18 @@ proc onSlotEnd(node: BeaconNode, slot: Slot) {.async.} =
 
   await node.updateGossipStatus(slot + 1)
 
+func formatForkSchedule(node: BeaconNode): string =
+  let consensusFork =
+    node.dag.cfg.consensusForkAtEpoch(node.dag.head.slot.epoch)
+  var res = $consensusFork
+  if consensusFork != ConsensusFork.high:
+    let
+      nextConsensusFork = consensusFork.succ()
+      nextForkEpoch = node.dag.cfg.consensusForkEpoch(nextConsensusFork)
+    if nextForkEpoch != FAR_FUTURE_EPOCH:
+      res.add " (next: " & $nextConsensusFork & ":" & $nextForkEpoch & ")"
+  res
+
 func syncStatus(node: BeaconNode, wallSlot: Slot): string =
   let optimistic_head = not node.dag.head.executionValid
   if node.syncManager.inProgress:
@@ -1550,6 +1562,7 @@ proc onSlotStart(node: BeaconNode, wallTime: BeaconTime,
   info "Slot start",
     slot = shortLog(wallSlot),
     epoch = shortLog(wallSlot.epoch),
+    fork = node.formatForkSchedule(),
     sync = node.syncStatus(wallSlot),
     peers = len(node.network.peerPool),
     head = shortLog(node.dag.head),
@@ -1982,6 +1995,9 @@ when not defined(windows):
       of "attached_validators_balance":
         formatGwei(node.attachedValidatorBalanceTotal)
 
+      of "consensus_fork":
+        node.formatForkSchedule()
+
       of "sync_status":
         node.syncStatus(node.currentSlot)
       else:

beacon_chain/spec/forks.nim

@@ -816,6 +816,20 @@ func setStateRoot*(x: var ForkedHashedBeaconState, root: Eth2Digest) =
   withState(x): forkyState.root = root
 {.pop.}
 
+func consensusForkEpoch*(
+    cfg: RuntimeConfig, consensusFork: ConsensusFork): Epoch =
+  case consensusFork
+  of ConsensusFork.Deneb:
+    cfg.DENEB_FORK_EPOCH
+  of ConsensusFork.Capella:
+    cfg.CAPELLA_FORK_EPOCH
+  of ConsensusFork.Bellatrix:
+    cfg.BELLATRIX_FORK_EPOCH
+  of ConsensusFork.Altair:
+    cfg.ALTAIR_FORK_EPOCH
+  of ConsensusFork.Phase0:
+    GENESIS_EPOCH
+
 func consensusForkAtEpoch*(cfg: RuntimeConfig, epoch: Epoch): ConsensusFork =
   ## Return the current fork for the given epoch.
   static: